howdy, im at a really frustrating point at the moment, ive been trying to take portraits of my own kids and some friends kids and i seem to be still getting subject blur in alot of images, sometimes the images look ok on camera but once loaded to pc look a little blurred or out of focus...what sort of things should i be looking at to correct this issue?

im working with a canon 450d with kit lenses (*sigh*) 18-55 and 55-250