Crikey me Claireg,
I'd be furious with these people, I know they left a note but the damage has alreay been done. The note didn't say if they were going to repair/replace the broken glass? It's such a difficult situation because in my experience if you try and approach the parents they tend to stick up for their kids, in my day I would have been made to apolgize to the person face to face then home for a telling off. I would ask the sectretary what was going to be done in light of the other complaints - it might put your mind at rest if you know something will be sorted. I take my kids often to the allotments and they behave really well, infact, other allotmenteers call them over, give them little jobs to do and they get strawberries and other goodies for helping. I never ever let my kids on other peoples allotments and keep a close eye on them. Good luck in resolving this matter,
best wishes
Eden