Kids Mattresses For Sale - Dora The Explorer Mattress, SpongeBob Mattress, are great little firm mattresses for kids. If you're having trouble getting the little ones excited to go to sleep then these firm mattresses by Serta may help a little. :) We're even told by the Serta company that these mattresses may be a hit with college students needing a cute mattress for their dorm room. The Kids mattresses are also water resistant.