One of the first things we installed in our popup camper was a collection of waterproof pads for the kids, who were still potty training (as shown by the wet pants hanging in the image above). This post describes some of the specific products we use to keep the mattress and upholstery clean and dry.
You’ll definitely want to wrap a waterproof pad over each of the slideout bunk mattresses. While not always a perfect fit, you should be able to order a regular full, queen, or king size mattress pad.
A flat crib pad works great for putting between your children and other upholstered areas, like the dinette when eating, or an upholstered bench seat. This does a great job of both waterproofing those locations and also making for a quick cleanup after the kids are done eating – simply take outside and shake (assuming you’re not in bear country of course).