Wombo combo is gamer slang for a devastating combination of moves, originally from a viral Super Smash Bros video. Often used in fighting games and metaphorically elsewhere. Not concerning. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in. Most slang of this kind moves through TikTok and group chats, so seeing it in your child's vocabulary just means they are plugged into normal teen internet culture.