Belisha beacon is the orange flashing globe atop UK pedestrian crossings, named after the Transport Minister who introduced them in 1934. Not concerning, standard British 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.