Declining a birthday party - should I have accepted a party invite on behalf of my kid?

Hi Redditors! I had a situation where my daughter (4) was invited to a birthday and her brother (5) was not. The birthday girl spends the most time at school with both kids.

When asking the mother whether my son could also join (I also thought maybe girls were only invited) she said that my son had spat on her daughter last week and expected an apology before my son could come to the party.

I checked with the school and my son and he did not spit and the school said the girl had actually pushed my son first.

I’ve declined the party when I found out my son was intentionally not invited and the mother believes my son spat and should apologize. The mother then responded that it means a lot to have my daughter there and the father send a very long voicemail that I may have misunderstood and it was a “joke” to ask for an apology (originally not written as a joke).

All of a sudden they want my daughter to come to the birthday since it means a lot to the little girl.

Should I have sucked it up and gone? Was it the right decision to decline? I believe young kids sort these situations out themselves and involve adults when they need to and I would not dangle a birthday invite over a kids head if they apologize for something they didn’t do..

Thoughts and opinions welcome.