I don't see any problem with calling this a virtual cache. It doesn't fit the bill of a "traditional cache" obviously.
As long as your description points out that there is no log book and nothing to trade, I don't see a problem.
When looking for caches to do with the kids, I see many that state that there is nothing to trade. Since the kids need the "reward" of finding a "treasure", we simply do not go looking for those types of caches.
In my opion, what you are describing is really no different.
