It WOULD be considered sexist if you put "boy" in place of "girl", RP. But that's because males and females have very different stations in life throughout the world. Boys already have the best opportunities that their respective countries have to offer (which isn't necessarily saying much for some countries, but still...). Girls don't. The Girl Effect isn't about putting boys down or giving girls a leg up over boys. It's about finally allowing the other half of the population to stop being disenfranchised, and finally giving them an opportunity to be active participants in the leadership and improvement of their communities (rather than just being obedient baby makers). These are opportunities that boys have ALWAYS had, even in the poorest of places. It's time to give girls a chance too.
And when the world finally recognizes that girls have just as much to offer their communities and countries as boys do, the planet's intellectual ability, talent, leadership skills, compassion, etc., will all basically double, because at last girls will have an opportunity to be USEFUL to their countries and communities on a large scale, and as leaders...rather than just having babies.
In short, it's not sexist when they use "girl" because GIRLS are the ones who are disenfranchised, devalued, and held down. Boys aren't...at least never to the extent that girls are.
Replying to pkulak's comment, I think that there have been in the past and probably always will be works of art that do the job they're meant to do in a very short time (even a few minutes or less in some cases - Some of Andy Goldsworthy's projects, for instance), and not just because we have cameras (now) to record these works of art -- A video camera recorded the making of this mural, for instance and so now anyone can see it at any time - It's will last as long as there's a copy of the video or prints from it. In this particular case, it seems to me, the making of the mural was as important as the final result (but of course it wouldn't have reached nearly as many people, if it hadn't been recorded...Still, it was indeed recorded and so it does still communicate something to us). - JV
I never understood people who do work that's so temporary. I suppose you could take a picture, but this guy is spending a lot of time and effort on something that's not going to be around for a very long time.