@Morph2lcfc, too many invitations (for my taste) for 2 younger kids (now y2 and 6) but barely any (a couple a year) for two elder ones (finished school, big age gap)
Never was bothered with the older ones. Now come to think of it must have been much rarer because there were no expectations placed back then on the parents to invite only a couple or the whole class, people just invited who THE CHILD wanted to see at their party.
For everyone who is doing Helen Lovejoy impression on this thread and pleading “won’t somebody please think of the children?!” and claiming any girl would be devastated to be left out of a classmate’s home painting party, it’s not true.
My kids 100% would not be devastated, would probably be relieved. We went to one of my daughter’s best friend’s birthday party in a church hall with a bouncy castle, it was a 2 kids joint party and the mother of the best friend told me she felt pressed into inviting the whole class (by the other mother). My DD heard it was everyone invited and immediately said she didn’t want to go.
In the end we only went because one of the girls’ was one of my DD’s best friends, turning 6 and we are sort of friends with the mum and I felt uncomfortable saying no.
My DD hated the idea of being a part of the writhing mass of screaming kids in the bouncy castle and actually spent most of the time outside on the lawn with a few kids like her who don’t enjoy noisy boisterous mass gatherings. She begged to leave after an hour, we left, lesson learned to actually listen to my own child and not to the voice of convention.
Anyway, what I’m trying to say, don’t assume everyone is devastated not to be at a party, plenty could be happy about not having to turn down an invitation! Or they might have better plans 
and couldn’t care less.
P.S. of course I always invited the kids via texting the parents direct or handing them the invitations privately.