Snug-fitting foam plugs the gap between bed and wall
Jonathan solved a the bed gap problem like this:
After our 2 1/2 year old transitioned from a crib to a real bed, my wife and I started were woken up several times in the middle of the night with our son in our room in tears. It turned out that the gap between the wall and his bed, about 2 inches, was just wide enough for piglet, his favorite stuffed animal and sleep companion, to slip down. All sorts of other objects would fall down there at the worst times.
I solved the problem by getting a 2" x 24" x 6' piece of foam (at House of Foam on High & Hamilton in Palo Alto, for those Bay Area folks). It fits snugly between his bed and the wall, comes up just to the bottom of the mattress (so it doesn't get in the way of changing sheets), and effectively traps tinkertoys, stuffed animals, books, and all sorts of other items that would otherwise fall behind the bed.