Chances are you have lots of stuff stored under your bed, but don't stop there if you are short on storage space. Look at any piece of furniture that has a skirt to the floor like sofas and chairs and use that area for storage too. Under the bed storage containers or old shoe boxes can be easily slid in and out. Think about things like wrapping supplies, photos, books, or anything else that will fit.
Under the child's bed would be a great place for that new toy that has more pieces than any toy should, like 450 piece Lego sets. Throw it all in a storage container and shove it under the bed. It can also be toted around the house easier too. Can a toy like a race track, train track, or puzzle be put together on a piece of card board and slid under the bed when you want the room to look clean? A bin under their bed will work for anything like barbies and dinosaurs and should make clean up easier for the child.