Yvon -
As for the humidity, womas are actually found in a wide range of areas across their natural habitat and will do just fine in a wide range of captive conditions. That said, most people keep them a bit to the warm and dry side.
Personally I bump the humidity up a bit when they are blue, and then change the paper and dry the cage out after their shed. The rest of the time, I don't really worry about humidity for them. (beyond that, any site that recommends keeping them at "high humidity" is not accurate and I wouldn't trust anything off that care sheet).
As for the substrate, Esp with a young woma, I wouldn't recommend any sort of gravel/sand as a substrate. I would stick w/ newspaper or paper towel for the time being for your woma. It is much easier to keep a clean and "healthy" cage for your animals.
I usually put several layers of paper towels in the tubs for the smaller womas - giving them something to get under.