I'm the adult...for once.
When a famous young musician is murdered, Castle and Beckett are on the case. Castle's daughter Alexis, a fan of the dead songstress, is horrified and wants to help anyway she can. But Castle gently/firmly lays down the law, telling Alexis to head to school.
An impressed Beckett almost didn't recognize this responsible parent possessing Castle's body. But as soon as Alexis leaves the precinct, Castle went right back to cracking wise and getting on Beckett's nerves. Some things never change.