Yummy mummy is informal British and Australian slang for an attractive young mother. Mostly used positively, sometimes objectifyingly. Not concerning, just casual descriptor. 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.