Google tells me that the collective noun for children is either "a tantrum of toddlers" or "an ingratitude of children". We just had Hayden's fifth birthday party, with our house filled with eight 5 years olds, and neither tantrum or ingratitude fits the well-behaved, but overly energetic, activity that we experienced, so I'll go with "an exuberance of children".
Lydia had everything planned: there was a balloon animal corner, a face-painting bay, a marble construction site, and a play-dough table. She even recruited Julie, assistant teacher at Hayden's school, to run a craft table. For us used to dealing with one fairly restrained child it got a bit much at times, but actually Julie was good at calming things down by forming a singing circle (where each child had a chance at being in the centre to sing a song).
Hayden had great fun, tempered only by Fatima not being present, and all the kids had great fun. The parents (in Europe, you just drop off even very young kids at parties) probably also had fun. We had fun too, in that exhausted everyone-is-laughing-no-one-is-crying-we've-nearly-made-it-to-the-end-without-a-disaster type of way.