I have been on both sides. Upset that DD1 was not getting invites, but then also being the mum choosing NOT to invite certain children. It is hard being on both sides.
Hurt for her about friendships maybe not being what we thought they were, also blaming myself, thinking it was due to me not being great friends with the other parents.
Then turning around and not putting invites into the pigeon holes of certain frenemies who DD was desperate to have at her party, as I did not want to encourage the friendship.

I think it is better for them to learn early one that not everyone gets invited, including themselves. As parents we need to learn how to deal with them being individuals along with the ups and downs of their friendships.