I'm not sure if he would have had a Robin in his Batman film.
I guess it would depend on the story he wanted to tell and whether or not he wanted Bruce to be the sole "hero" in his rookie days, or whether he wanted to play on the mentor/ father figure aspects of Bruce Wayne.
As far as an All Black costume goes, I could actually see Kubrick being down with the Ringwood like Black costume from 89.
I could also see Kubrick going for a 1939 style Batman costume to emphasize the more pulp origins of the character. Maybe it would depend on what period Kubrick was making the film.
Danny Lloyd was a great child actor, so I wouldn't have minded seeing him as Grayson.