When my daughter was around 6 or 7 we were going through the birthday party phase. First, if she had a party, then everyone in her class, Sunday school class, Brownie troop, and dance class was invited. Of course I knew not everyone would come, but I was always one who tried to include everyone - at least when they were that age! After the party was over, I made sure she wrote at least one line to thank that child for the specific gift, and one line to thank them for coming. And she did.
We went to another classmate's party once. Again the whole class was invited. When the party was over, the mom stood by the door with the "party favor bags." When DD got to the car and opened her bag she found an envelope. She opened it and it was a generic, but handwritten, thank you card saying "Thank you for coming to my party and thank you for the awesome gift." Never saw anything like that before or since! 