Dirty soda is a Utah-origin trend where flavoured syrups and cream are added to soft drinks. Popularised on TikTok. Not concerning, just a drink. Often associated with Mormon LDS culture (alternative to coffee). 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.