I’ve said it on a thread like this before but I’ll say it again.
When I was a child all children were left at parties, it was not expected for parents to stay and probably would have been weird tbh. A lot of parties were held in people’s homes with party games and such, there simply wouldn’t have been space for everyone’s parents as well! We also loved the independence from our parents for a while.
*I’m not spending a portion of my weekend at someone else’s kids birthday party so yes, I leave my DC. I also have other DC and don’t always have childcare so unless they want to accept my other children as part of the party then...
It’s absolutely fine and imo NORMAL to leave children at a party. Maybe not under four but once they’re reception age it’s fine*
As you say, that's your opinion.
IMO It isn't normal to leave 4,5,6 year olds in a public place for one or maybe 2 people to look after. That isn't safe.
You're not spending a proportion of your weekend at some other kids birthday party? That's called being a parent! .
If you have childcare issues, decline the party invite or pay for your other child to enter the place where the party is at.