There is no law keeping you from photographing anything if it is in or on a public space. At least in the US anyway. However, I would guarantee you a visit from the local policeman if you were to start photographing children on a school playground for instance. You would probably get a request to leave.
The problem comes when a person has an expectation of privacy. No matter where they are for instance in a public restroom. I suspect the UK has similar laws but you need to check.
In a word, photographing other people's children is not a good, or perhaps healthy, idea without the parents permission.
EB
EOS 1D, EOS 1D MK IIn, EOS 1D MK III, EOS 1Ds MK III, EOS 1D MK IV and EOS 1DX and many lenses.
