When I was a kid, eons ago, thank you notes were a rarity. In fact, we never got one. All of the thanking was done at the party, in person. (There's a Seinfeld bit in here.)
This is the first party we've held, so I'm trying to avoid a social faux pas.
In the past, when my sons have been invited to parties and brought gifts, sometimes we received a thank you and sometimes we did not. It was no big deal either way.