Kway teow is a flat rice noodle used in Southeast Asian cuisine, especially in dishes like char kway teow. Not concerning, just food 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.