Yay, your child was invited to a party of a classmate! They are accepted and included! How about acknowledging the kind gestures that follow from interacting with others in the world, with a short little thank-you note:
I just wanted to thank you for seeing that Ryan has such a wonderful time at Rory’s birthday party. It’s not easy being the youngest child at a party, especially when all of the other children are at least two years older and considerably bigger. By giving Ryan a little extra “help” with the piñata, you really made him feel like he belonged with the big kids. He told me he had even more fun at Rory’s party than at his own!