Oooh...quite a tricky situation. Of course kids should be allowed on allotment sites but shouting, going on other peoples sites and vandalism is obviously not acceptable.

How about asking the supervisor to put a sign on the entrance gate that 'children are very welcome but need to be supervised and keep noise levels down'.

I always find that if you know the children, then you find their noise easier to deal with. Maybe the window breaking will give you a chance to get to speak to the children and get to know them a bit.

I wouldn't be surprised if the parents haven't already had a stern word to the children about the glass breaking and they are probably are a bit nervous about seeing you now.