A.M.
I would get a regular mattress for them because they can use it for years to come.I have a 4 yr old and we got her bunk beds. One mattress is a Serta it is one the bottom and the other is a more inexpensive one it is on the top. Sometimes my mother comes over and she sleeps on the bottom that's why we have a good mattress there, My daughter doesn't seem to mind the cheaper one, she likes it because it is bouncier than the Serta. She has a waterproof cover on her mattress. They make them so they are just like a sheet; soft on top and you can put a regular sheet on top. Hope this helps.
-A.