Aegyo is a Korean term for a cute, charming, childlike demeanour, often performed by K-pop idols and Korean drama characters. Cultural performance. Not concerning, just Korean vocabulary. 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.