Anne and Peter have become much better friends now. They usually have meetings in Peter's room to talk about their emotions and feelings. Tonight happens to be one of those nights.
All the adults believe there is something more going on in Peter's room then what Anne and Peter say to be doing. Mrs. Van Daan in particular doesn't appreciate how much Peter is paying attention to Anne and not his mother.
Before Anne leaves, as told to at 9 pm, she asks Peter if he's ever been kissed before. They talk about their experiences and how they wouldn't consider them real. As Anne leaves, she gives him really strong signals for him to kiss her and she thinks he doesn't see them.
They must've kissed!
In fact, Peter does see her strong signals and just before she opens his door to leave, Peter kisses Anne on the cheek. She leaves his room dazed about what Peter finally did.
Anne is so happy about her kiss from Peter, as she goes around to say goodnight to everyone, she kisses them on the cheek.
Once Anne gets to Mrs. Van Daan and kisses her, she gets suspicious about it. She then realizes what happened between Anne and Peter!