Do you assume it's ok to also bring siblings? Does it depend on where the party is being held?

I'm planning my son's 3rd birthday at a gymnastics gym. We want to invite his whole daycare class. There are 24 kids in his class and we can only have 25 at the party, plus we have friends with kids we'd like to invite. I know not everybody will come but I don't want a bunch of older siblings instead of friends.