If the boys want to come to the party, welcome them. They'll probably love making the horses and many of the other games. Teaching 'knightly conduct' would not be a bad thing. The little guys might actually enjoy it.
Call them 'Courtly Manners' and you'll have them all ready for Ren Faire when they grow up! The boys could learn how to bow and the girls learn to curtsey, and they could all learn the basics of general manners.
No, I do not have children, but I have been accused of acting like a child more than once.