It is often easier, when you decide to go hunting for mushrooms, to pick up a kid first, then drag him/her around the likely spots for them to grow. As charley says, when you spot one, press the spacebar,drop the kid on the mushroom, then drag as may more kids as you can on top. When the action line stops saying "picking a mushroom" the mushroom has disappeared and you can unpause the game and let them take the mushrooms to the bin. Don't move any of the kids once they've been dropped until they've delivered their finds to the food bin.
One further tip: if you have the GC try scrolling around the screen holding him. It stops him from throwing parties at awkward times, and makes sure he doesn't cause all the other kids to drop the mushrooms on their way to the bin.
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To err is human; to arr is pirate.