I was going to ask about this today, I'm glad to have found your post.

Sometimes the kids will pick up a mush or collectible and not move, then the item disappears.

Sometimes they "run into" an object and spin until they have lost the item.

I find that after I help them pick it up, I move the screen so that the kids aren't seen. (kwim?) I find that they run into fewer things that way.

I keep telling myself....MY REAL KID DROPS STUFF ALL THE TIME, sooo maybe it's a feature not a _bug_. LOL!!!