For DD it would be figuring out how to hug her big brother. Cue heart melting. Or recently she's learned about "all fall down" for 'Ring Around the Rosie'. The looks she gives me when she falls down? Priceless.
For DS it's watching him hold DD's hand or the way he says 'Thank you...momma..." Very precise, very haltingly. That little voice kills me.
What about you?