It’s interesting that this line works on Laertes. There is a tiny hint of patronizing in it – calling Laertes a child really shouldn’t work. But I suppose in reminding Laertes of his status as the child of Polonius, the aspect of calling him a child gets subverted and Laertes has to accept the compliment of being a good child or deny being a gentleman. And this is how Claudius wins him over.
