Let's put the scene in context. The daughter and her friend share a secret. The reader knows what it is. The mother doesn't. The reader knows exactly what the daughter is thinking. It's not necessary to say she is "thinking about something." A funny or naughty incident might have occurred. Describe her expression. Maybe she winks at her friend. Her lips are sealed. The secret's safe. Or maybe the mother already knows what happened. In that case, uh oh. They're in big trouble.