I think your all taking this too far.
The officer had taken down their details that they volunteered to give when requested. there is no indication that he is blacklisted as the first suspect if anything happens in that area.
They are probably keeping a list of everyone they catch in large groups, and if they keep finding the name crop up again and again they can do something about it.
Remember, the police informed you of this proceedure, they are not keeping you in the dark.
As for the police officer sending their kids home, why cant they? they are his kids and maybe the officer knew some kids in that group who he didnt want his kids hanging round with (Like any parent would do)
Police officers should be commended for taking such action, with the amount of flack they get from every little thing they do this was a brave thing for them to do.
In my own community police officers wont even arrest an immigrant (Our city is flooded with them now) as they fear that they will be taken to court for being predujice EVEN THO there have been alot of rapes outside our nightclubs allegedly by these immigrants.
I just think any step a police officer takes to make the community safer is a good thing and they shouldnt come under scrutinity for it, they let you know what had happened in your letter so whats the problem? if this group of 15+ kids were on a football pitch im sure no one would of minded, but a group of even 5+ kids i'd find intimidating and at 6ft4 im not the smallest person in the world.
If you dont want those police officers tell em to transfer over here, id welcome them with open arms