There is no reason a toddler can’t continue to use their crib mattress up until they move into a full-size bed. “I usually advise parents to keep their children in the crib for as long as possible, especially if their child has no problem staying in there,” says Dr. DeBear. Toddler-size beds are typically sized to hold crib-size mattresses and it’s perfectly safe to simply move the mattress from the crib to the bed. In fact, Dr. DeBear points out that if you have a crib that converts into a toddler bed and you already know the mattress you have fits, so there’s no reason not to keep using it.