AIBU to think this is quite odd behaviour?
They are such good fun, loud, energetic that most of the children in there were chasing after them and playing a tickle monster game.
The main issue I have with this I think is that he gets all the children so hyped up that they start to act a bit naughty, ie pushing each other, grabbing toys off each other etc.
If my dc and my friends 2 dc were just playing calmly together these things wouldn't happen very often at all.
All the children playing with the other parent were under 5 and they only had one child with them, the others were other peoples, probably about 9 others.
This other parent is always there on this particular day and me and my friend also go on this day every week too.
It has got to the stage where ds said to me today, look mummy there's my friend, which seems a bit over familiar as they have only said hi to me and my friend a couple of times.
I am more than happy to be told that IABU.