Depending on the progress of the village there are certain areas that the kids tend to hang out and those are good to check first: by the cactus; by the rock; by the ruins; in the field/garden. I also use the Map key/button to help quicky spot a kid and zoom in on him, but will more often use the school drum to gather lots in one place.

This morning when I was checking on my villages before work, one of them had the "pox" event and I was busily curing the kids in the "clinic" area when a mushroom popped up right there!

Putting the game on pause, I was able to get 29 kids on that mushroom! It didn't matter that many of them were still "not feeling well." That was the most I every got in one shot.

BTW, I once seen a pushroom pop up near the laundary hut while I had a kid bring another mushroom to the bin. So there doesn't seem to be a minimum time between mushrooms. But it does seem like collecting one mushroom will make other pop up sooner.