In VV2 the two oldest kids will fail to pick up a common collectible (or a mushroom). Therefore you need a minimum of four kids to get the collectible more than once.

If an uncommon or rare collectible appears for the first time all the kids (including the oldest ones) will be able to pick it up, but once it appears again it will be treated like a common collectible and the oldest two will fail.

VV3 is different. I've never managed to get more than four on a collectible, but I suppose it is theoretically possible to get more if you are very fast.