"But Dad," the boy tells me, "It's okay, everybody plays with unpainted terrain!"
"Not in this house," I bark at him. "So long as you live under my roof, you'll play by my rules." Which is a bold faced lie, as 40K is anything but my rules. We'll let him slide on that one, but some things are simply non-negotiable - like using unpainted terrain.
These are the painted up craters that we built together a week or so ago. He's off at something called, "Winner's Camp" (which I was excited about until I learned it had no relationship to tabletop gaming) learning how to be a good citizen and apply himself and work well with others by rock climbing and doing other team building exercises. While he's gone I decided to be a good citizen and apply myself and work well with others by painting his craters for him.
Lead by example, sometimes that's all a parent can do.