It is only for your son, and/or, you.
At this age, often times, the parents are allowed to stay. Versus it being only a "drop off" party.
Keep in mind, that all parents have budgets. And per food/party supplies/goody bags etc., and the party size where the party is held.
IF however, you do not have anyone to babysit your daughter, then you can ask the party Host, if you may be able to bring your daughter, explaining why.
When I have parties for my kids, it is for their invited, friends. Only. AND yes, all of my kids' friends, have siblings, whom my kids know and play with too. But at no time, do the parents assume, they can bring all their kids or their whole family to the party... just because they know us and our entire family. They only bring, their invited, child. And they have their Husband or a grandparent, watch their other kids. They don't bring them all, to the party.
IF your daughter were invited, then her name would be on the invitation too. And if you bring your daughter anyway, without checking with the Host if this is fine, then that is very rude.
Again, the party... is only for the invited child.
Also, when there are siblings in a family and one of them gets invited to a party... that invited child, should be able to have their own things to attend to too, without their siblings going too, all the time.
ie: I have a friend with 3 kids. We ALL know them and ALL their family and my kids are friends with all her kids. BUT, when my son had a party, he invited THAT sibling, which was in his class. And the Mom... ONLY brought her child, that was invited. Only. She also said, that her son had a right to go to his own party without all his siblings all the time, to have his own... special time with his friends. And then her Husband watched her other 2 kids, at home.
There are many people with more than 1 kid. But that does not mean, that every time one of the children gets invited to a party... that ALL the children in that family, can go too.
The Host/family of any party, has a budget. And costs, and they also have a limit as to how many or what size party, they plan.
They cannot possibly, get food/supplies/goody bags for every sibling that tags along to a party in addition to the invited child and/or the parent that is invited.
It is your son, that is invited. To this party. For HIS, kindergarten classmate.
When a person just brings along others to a party, it is rude.
Recently, my son was invited to a party for another boy. We know their family very very well. They have a daughter too. So my daughter asked if she can go too, to play with the sister of the boy. I said, no. The party is for your brother... and the siblings are not invited. Even if we know them very well.... it is your brother, that is invited. And your brother has a right to go to his own parties, on his own too. With his friends. Sure, I could have asked if my daughter can go too. Because I know that family VERY well and our kids know each other VERY well. But I did not. The party was for that family's son and my son, was invited. Not our whole family. And that family, ALSO explained to their daughter that the party was for her brother. It was his, special day.