First of all, I think what you are doing for charity is spot on, hats off to ya !!!
As for doing gigs for free….. It defiantly depends on the situation. When I first started out, I was working in a bar, and covered the resident DJ when he was on holiday etc…. I got paid my normal bog standard bar wage, but I was happy as it was a vital learning experience. I also put on my own club night to advertise my self (which i did for free).
However, now that I have a residency, I don’t think I would do a gig for free. Simply down to the fact that my music and my equipment doesn’t pay for it self.
However, I have done charity gigs myself and I wouldn’t charge for them….as long as it’s for a good cause….. Just like the one you’re doing 🙂