Pick a deep drawer so the depth change won't be obvious. Cut 1/4-in. plywood 1/16 in. smaller than the drawer opening and rest it on a couple of wood strips that are hot-glued to the drawer sides. Then hot-glue some item you'd expect to find in that drawer to the bottom so you have a handle to lift the false bottom and reveal the booty.
Kid's Room Hideaway
No burglar worth his salt looks in a kid's room for valuables. It's just full of useless junk. So find somewhere in there where the kid won't find it either.