[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.