Fake it 'til you make it. The theory is that going through the motions will eventually inspire prosocial behavior with or without any initial feeling attached.

Okay, that's a little glib, and there's some evidence that it doesn't work too well when it's externally imposed. But those studies (Cialdini cites some, for example) were generally done on adults, and it might work better on children; alternately, it might be more about inculcating the forms of prosocial behavior and trusting that they'll get hooked up to the right emotional content later, when kids' empathetic faculties are better developed.