i know they make all kinds of greases with specialized uses, but come on. these are mopeds. set it, and forget it. no need to get exotic.
besides. wheel bearing grease does very little to directly reduce friction. the bearings themselves reduce the friction. you can test this by spinning an ungreased wheel with new bearings and a little chain oil. spins better than it does with grease in it. the grease itself is a source of friction to some extent.
but its all about reducing heat. heat will cause expansion which will cause REAL friction (metal scraping against metal)which will cause failure. grease serves to spread the heat around thus preventing expansion, thus preventing friction.