I was a cheerleader before I joined robotics, and now I'm on my team's build department. One of the things that my team does when recruiting is focusing on the fact that there's something for everyone. Knowing that no matter what I wanted to do whether it was wiring, building, taking pictures, graphic design, strategy, or whatever there was something for me. I would suggest really focusing on the fact that there's something for everyone even girls. That usually is a good selling point and can attract more girls.
I would also suggest having your girls register for
FIRST Ladies, this is a great resource to have your girls currently on your team talk to other girls in FIRST. It would also be another great way to get advice for recruiting girls.