Nepo baby is short for nepotism baby: a person who got their career boost from famous or wealthy parents. Often discussed about actors and models. Not concerning, just industry commentary. 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.