You can use treated (exterior grade) wood. It is no longer impregnated with the arsenic-based chemicals of years past (safer now). Or you can just stain it. Overall I think this is a good idea although to echo some of the above comments yes, I'd strongly consider building in some kind of access door. You can always put a lock on it.
Is your intention to keep the deck in place permanently or just until the kids get old enough to not potentially drown themselves by falling in a few years down the road?
It's a perfect place to hide a surreptitious grow room too.
