I was invited to a party last year, where the date and time was changed THREE times.
That really annoyed me at the time, and I can imagine, that if my son (about 10 months old at the time) had been older and had consciously been looking forward to this party, it would have been even worse.
So unlike anybody else here, it seems, I can feel with the mother, she now has to tell her little boy he cannot go to the party, which might be a big deal at four years old.
I think you should have checked with the best friend first and then sent out the invitations.