I think getting kids into cycling is FANTASTIC!!! Not only do they get to try out a great sport, but they get introduced to a healthy lifestyle.
However, you had a HUGE group ride on a MUP, and whether it is a group of adults or kids, I don't think MUPs are the place for a ride of that size, because sharing the path safely can be a major challenge. I would recommend bringing smaller groups of kids out at a time and making sure that they understand the importance of respecting the center line, etc, and you should be fine.
Good luck!!!