I'll make some suggestions(but with the caveat that I have no idea what camera you're using!!)
If you're totally new to photography(and are keen about it) then forget Av and or Tv mode, and manual mode for that matter!
Again I'm basing my recommendation not knowing what
lens(es) you use or have access too as well. So I'm going to assume that you're using kit zoom type
lenses. 18-55 or 18-135 or something like that.
Because you mention Av and Tv mode, I think it's safe to assume a
Canon camera.
So: for kids, stick to Av mode. Set
aperture to the lowest number and don't let it move from there.
Set
ISO to Auto
ISO, but don't just leave it there without further tweaking that setting(if possible). In the Auto
ISO settings, you should have an option for an upper limit value. By default I think most if not all cameras will be set to the maximum
ISO number possible.
So if it says ISO12800, then change it to 6400. Assuming a modern
Canon(eg. 70D or 80D or similar) then ISO6400 won't be an issue unless you need to crop heavily.
Just be sure that there is no
exposure compensation set in camera with those settings above. ie.
exposure compensation set to 0Ev.
So camera settings will be something like:
Av mode
Auto
ISO with a maximum ISO6400
0Ev compensation.
Now not knowing what
lenses you have, the longer the focal length, the faster the
shutter speed needs to be, with or without IS. IS helps in some situations, and does nothing in other situations.
if you're kids are running around like small twitchy birds, then IS will not help.
If they are moving about, like playing with sand in a sand pit, or down a slide, IS can help.
This depends on the
lens being used too tho. At shorter focal lengths(say up to about 70-ish mm, IS doesn't help as much.
If you're
lens can do 200-300mm, then IS helps you hold the
lens steadier at slower
shutter speeds.
So even if the kids are moving about quickly and you need say 1/100s
shutter speed, but your
lens is set to 300mm, 1/100s may
stop motion blur of kids moving, but you're at the very edge of hand holding a
lens at 300mm and 1/100s. This is where the IS helps.
The other aspect of AutoISO usage is the minimum
shutter speed set. This tells the camera at what speed to start increasing
ISO. It's not an easy value to set either. Once again it's dependent on the
lens type you're using.
But what this
shutter speed value does is that it's the minimum
shutter speed where the
ISO is increased to maintain that
shutter speed.
The important point to note here is very simple: it's far more important to most images to have a
sharp point of focus(ie. due to
shutter speed) than it is to have the lowest noise grain quality at the risk of focal point blur.
Many people think AutoISO is some kind of evil feature best avoided, but I can tell you it's probably the best feature in this modern age of digital cameras. (note that older age digital cameras had woeful high
ISO quality compared to what you have in the modern era!!)
So your options are to set
ISO as low as possible and get nice noise-less images of interesting blur .. or slightly grainer images of something interesting and clearly in focus. minimum
shutter speed choice is important. longer focal lengths = faster
shutter speeds. Don't forget to add iin the IS variable too tho!
So if you
lens is a 15-85mm
lens, maybe use 1/60s as a minimum
shutter speed. If your
lens is a 70-300mm, then maybe use 1/100s as a minimum
shutter speed. it varies according to
lens used.
I don't know how Canons have AutoISo set up, but some cameras have a 1/focal length
shutter speed for minimum
shutter speed. If your
Canon doesn't .. it's a great point to illustrate why you really wanted that
Nikon, but didn't know why!
Anyhow, if you can use a 1/focal length value for minimum
shutter speed, then awesome, as it doesn't matter what
lens you use .. the minimum
shutter speed will vary accordingly. so at 50mm it'll automatically set to 1/50, and at 300mm it'll set to 1/300s.
Not only that, but also allows you to factor in IS too(in
Nikon speak that's VR). So if you have a 300mm
lens with IS(VR) then you can adjust for that, and it'll automatically set to something like 1/150s or 1/80s.
FWIW: I can't remember a time when I've used my cameras handheld when I haven't used Auto
ISO. On a
tripod, it's silly to use it .. you just don't need it unless you want extra
shutter speed there too, but handheld I use it all the time, and even back to the crappy old D70s days where ISO400 looked dreadful!! But like I said earlier .. better a clear image with noise than a mess of mush with super quality grain level at low
ISO. I don't pixel peep blurry images, and only inspect my sharper images with the view to keep the better versions.
Hope all that makes sense, because there's more! .. and this may be the more important point to be ware of.
Focus mode and focus point usage.
It's probably more important than any of the above. The above suggestions only help if you can get a clear
sharp rendering to begin with.
Many cameras come by default set to auto area focus point mode, and use what's known as Single shot focus mode.
That is, the camera wants to choose which focus point to set, and it only focuses and then allows the
exposure to be made. You hear the focus beep and shoot.
BAD!!! .. big bad BAD!!
Use Single point focus mode. And use the central one when
chasing kids. If your shooting kids playing in the sand pit(ie. not moving about much) you can vary the focus point to get better framing(framing is not only important, but vital for a pleasant looking image).
And set the camera to use continuous focus mode. I don't know what
Canon call it(maybe 'servo') but it's where the camera always focuses and refocuses, and focuses again and refocuses again. That's why it's called continuous focus mode.
In single shot focus mode, it focuses once and then it takes the shot. In the micro second between focus and shot, the subject(or yourself) may have moved enough to be slightly out of focus in a fast paced shooting situation.
In continuous focus mode, it's always re-focusing up until the point of
exposure. There's a better chance that if all the other aspects of the setup are OK, then you won't have slightly misfocused at the point of
exposure. (actually it's not the operator that will have misfocused, it's actually the camera that does.
if the scene is more static(like the sand pit playing situation) then the difference between continuous and single is less obvious(if at all).
Lastly, for kids moving about a lot and quickly, shoot multiple exposures in rapid shot mode. If you camera does 5fps, then good. if it does 7fps .. better. Use it to advantage.
So the point of all this is .. don't worry too much about
one single aspect of the cameras settings. It's more important to figure out a selection of settings that work well as a whole. Yes those single elements make a difference, but if the other settings aren't helping, that one setting that becomes a discussion(or sticking) point may not help at all!
So my suggestion is to set up the camera
and to recap)
Av mode
AutoISO max ISO6400
Auto
ISO min
shutter speed(depending on
lens focal length)
AF mode: Single point focus mode and Continuous(servo?) mode. (keep the
shutter half pressed as you track the subject and always be ready to shoot .. and as side note, you may have an option to use 'back button focusing' which also helps)
high speed frame advance mode .. max it out.
What will this do?(assuming a
lens like a 15-85 f/3.5-5.6 or similar)
it
could .. if you track the rapidly moving subject, holding the
shutter to half way down, it will continuously focus on the subject as you track, You're using single point AF mode and you keep the af point at the subject. You have a 15-85mm
lens at set the
aperture to f/3.5 at the wide end. Even tho you zoom in to 85mm, the
lens will stay wide open at f/5.6. You set the AutoISO to 6400 and 1/60s min
shutter speed.
Light is good for 1/60s and
ISO 200 at f/5.6. Even tho you set AutoISO to 6400, as the
light is good,
ISO will stay put at 200, as your minimum
shutter speed is still only 1/60s. Shoot a set of 5 frame at 5fps as you follow the subject. Now the subject moves into the shadows, and
light drops. You have touched nothing on the camera, no fumbling for settings just keep following the subject. Your only concern is following the subject, forget the camera settings now(unless you change
lenses to something really different!!). Now
light levels are 1/60s ISO1600 at f/5.6 at 85mm. But you change the
lens to 50mm as you stepped a bit closer. So
aperture changes up to f/5.
Camera knows all this(because you set it all up earlier, and have touched nothing since), so
shutter stays put at 1/60s,
aperture now moves to f/5, so
ISO is set to 1250(not 1600).
ISO is set to a minimum value as possible. Because it needed 1600 at f/5.6, you moved the
lens to 50mm which set
aperture to f/5, so
ISO is adjusted to suit(ie. ISO1250).
If you go back out into the
light again and say
shutter speed is now automatically set to 1/500s as the
light is much brighter than before,
ISO is set back down to ISO200 automatically again as the
shutter speed is way above the 1/60s minimum we set in the camera for AutoISO.
Hope all that made some sense too.