I have been on the 'other side' of requests and there is, you've guessed it, a form for everything.
As long as all parents/guardians have provided signed permission for the images of the children, the LA is cleared of their responsibility of ensuring the 'safety' of the children, as far as images are concerned.
Unfortunately, either the attendant did not know about the requirement, the particular amenity did not provide the appropriate training for that situation or the appropriate documentation, or the attendant thought it easier to prevent the photographing than to adhere to the correct procedure, or he was unable to leave his post in order to comply with procedure, we'll never know.
The difficulty is of course, obtaining that signed permission from the parents of any/all children that might also wander in to shot.
Next time, try picking a quiet time when there are few or no other people likely to wander into shot, (not easy I know) and prepare yourself by discussing it with the manager first and ensure that you have the signed permission of all relevant parties. The manager might have appropriate forms available.
Yes I know, it's a pain. Data protection laws have more flaws than benefits imo. Sign of the times; rushed, topical, popular regulation!

Incidentally, sorry about my assumption re: ranting.
