In general children are afforded no more protection than adults, there are odd and rare exceptions regarding children under protective orders
It is not illegal to photograph anyone in a public place
If there are terms and conditions of entry that preclude photography the event manager may ask you to leave and if you don't you could be prosecuted for trespass, not for taking photos
Any request by say a football club for you to get permission from parents or managers is just their guidelines and that's all it is, not enforceable by law
This advice is not applicable to commercial use, that's another issue again and will usually also, at least in Queensland, require a blue card