I really want to see a flashback from a kid Grooberson's perspective. It might be the easiest way to shoot a flashback that avoids the uncanny valley effects of de-aging.
Let's say a flashback has kid Grooberson hiding from a ghost . The Ghostbusters have been called are on on their way. Shoot the scene from the partially obstructed vantage point of a child. Keep the camera low to the ground and make it difficult to see faces. Have the Ghostbusters kick in the door, and shoot the scene in a way where you only see characters from the neck down or the back (show off those proton packs) as they bag a ghost. Basically... use the
Muppet Babies trick:
Hell, maybe the kid doesn't get a good look at the Ghostbusters' faces because he's fixated on the trap being deployed, and makes the mistake of looking directly at it.
Dub in some unused audio from the first film, include a shot of a smoking trap, cut to a scene of Ecto-1 driving off, and call it day. No Tron Legacy CGI people issues.