It does almost sound like they have a don't ask/don't tell secret pact of some sort. That still doesn't make it right. I probably would call with the idea of confirming the numbers for care because I was at (and name the park and the time I was there) and it was obvious that several other caregivers at the park had at least whatever number of kids in their care. Then you can thank them very much for confirming the number of children allowed and that that is what you have in your daycare. At least you have given the information to them and really no one can say the call was to get anyone in trouble because nothing about what they will record as the complaint indicates that - it was all about confirming the rules. The fact you dropped the info they would need to find some people not in compliance is well just a bonus.

But I too would feel responsible if anything happened and I knew the event was taking place and did nothing to protect the children.