Well, when I DM a game for my son, he has fun. He likes playing powerful characters. When he begins to 'power creep' I just simply give him and his group a challenge that saps them of a lot of their gear and never pull punches with the player deaths.
Also, by the time they're facing an Ancient Blue Dracolich and her horde of gnome militants, the fact their rogue can cast spider climb does not ruin the game.