I say that too and I scrabble around trying to find one only to find that by the time I've found one and got them there the mushroom or whatever has disappeared so then I say something very bad indeed :(.
I picked up a tip from someone else on the forum, which works fantastically. You just pause the game with SPACEBAR, take your time to find a child, drag him/her over the item, and it will state "collecting blah blah...". Then once you unpause the game, the kid will kneel down and collect.