The rules in very simple black and white for England are so…
If its private property, you are controlled by whatever rules the people that own that property set or the ones that have hired it, no matter how sensible or stupid they may be. There is no variance at all on this.
If its public property, you can take pictures of whatever you want period. It can be football matches, girls’ hockey practice or kids playing in the local fountain. It doesn’t matter if its school organised, a private group or just happens to be some place were some children can be found together. Legally the only thing that can happen is the parents remove their children from the area so as they are not in any photos.
The problem is that if you were to publish any photos, if they contain others that are recognisable and you do not have written permission from them (or their guardians if they are under 18) then you are breaking the law.
A little story to back this up, a friend of mine and myself were doing photography in the centre of Manchester last summer, there are a load of fountains/water jets that all the children play in during the hot weather. We set up there to catch the enjoyment of these children and were minding our own business; there were many other people with cameras there also taking photos and obviously many, many people around the area, some with children and some without. Within about 10 minutes of us being there we were approached by a policeman that said he had received complaints about us and asked what we were doing. As I had nothing to hide and knew my rights I explained exactly what we were doing and offered to let him view my pictures. He said that he believed us but asked us to move on. A long discussion developed and in the end he conceded that there was nothing legally he could do and it was the parents that were in the wrong, however he was just one policeman and there had been complaints by several parents. If we didn’t move and the parents decided to act themselves although they would be breaking the law, he felt it would be in everyone’s best interests that we moved on.
The moral is that although you may be correct and have the law on your side, it doesn’t necessarily mean that it is wise to do it.
HTH