In VV2, I've found that you need to pile three kids onto a mushroom or collectable and you'll get two mushrooms or collectables. It's kind of strange, but if you put only two children on it, you'll still only get one.
In short, in VV2, you need to follow this formula: The number of mushrooms or collectables is equal to the number of children minus one.

(NOTE: This doesn't apply to a collectable if it is uncommon or rare and hasn't been found before.)