[SPOILER] I found it. This was originally posted by Swedane (see post #156394: Once you see a collectible; 1. Pick up a child and pause the game before dropping him/her on the item. 2. Keep the game paused and pick up a second child 3. Hold the second child over or close to the item. 4. Unpause the game and quickly drop the second child onto or near the collectible.
Loc: Right behind you.
You can actually triple any pickup using this method. [spoiler]If you have three master farmers and a chief, by dropping them on the fire to dance you can also make it rain at any time, which will help your mushroom hunt.[/spoiler]