I had a little mermaid party when I turned 5. I invited 4 or 5 boys, and of them, only one was able to come (dangers of having a birthday in the shadow of Christmas...) He loved it, and to this day (20 or so years later) will jokingly bring up how he was Prince Eric when other girls were regulated to roles like seaweed.
I was one of the girls who had both male and female friends and never saw an issue in having to segregate them. Go ahead and invite the boys- they will come if they want to, or not if they don't. As we always say here, an invitation is not a summons.