I have experience with summer camps for Mathematics (as a participant) for kids from 10-18. I'm not sure how they manage to achieve it, but it was probably 55%/45% male/female. I think that half the camp elders being female helped a lot, as well as wise promotion and word of mouth after 20 successful years.

I feel that the camps being mixed added a ton to the experience. I can't begin to describe all the benefits, but for me as a boy, the camps first showed me that both genders can be interested in nerdy stuff, and that such interests are actually fine and quite normal.

However, if it would have been 95% boys, I think it would have been better to split them by gender. Especially among children it's hard to be the only girl in a group of only boys. Then, once the girl group starts to grow by intelligent promotion and word of mouth, perhaps you can merge the camps once the split is more favorable.