The pensive face emoji conveys quiet sadness, disappointment, or thoughtful resignation. If your teen uses it occasionally it is just expressive; if they use it constantly check in about how they are doing. 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.