Instead of creating a land portion that is water resistant, you may just want to buy a plastic aquarium divider. That way you can have a loose substrate. Spot cleaning is probably not good enough... even when picking up giant White's poops there is still bacteria left behind, and when in a warm humid environment can grow into a mold or something dangerous to your frogs. I recommend changing the substrate, actually taking it out and replacing it. If you don't care about aesthetics, than the best substrate, and easiest with the type of set up you are talking about, would be paper towels.

I don't have much experience in building vivariums, but if you are talking about a set up for White's (by the way, Dumpy's and White's are the same thing) than be sure there is an easy way out of the water, like a small pile of stones to one end, because they are poor swimmers and could easily drown if the water is deep enough.

Hope this helps, maybe a little!