Smileless
It is possible (and recommended) to distinguish between a child's behaviour and the child him/herself. For instance, you can say "your behaviour right now is silly/selfish", which is different from telling the child, "you are silly/selfish". I'm reminded of the line from the film "Raising Helen": "By the way, you're not a bad person, but this is very bad behaviour".
The idea is that if a child is constantly told he/she is bad, stupid, selfish, silly, etc., he/she will eventually start to believe it. This can lead to the child acting out or not bothering to try new activities, based on a belief that he/she can't change. The idea is, "Well, if Mum says I'm bad/stupid/selfish/silly, I must be that way, and so it's okay for me to behave that way. I can't help it." If you identify that the behaviour is problematic, the child can change his/her behaviour.