I have pretty fine hair, definitely not thick at all (I wish) and I have always kept it long. On the contrary, I find it has always been easier to brush as opposed to my sisters thick full hair. Hers is a real project to brush, she used to cry when our mom did it and she would ask me to do it instead because I was gentler. My hair has never given me any trouble being brushed or put in a pony tail, it must be a texture thing or something haha.
I guess you'd just have to keep it simple until her hair gets a little more full, you know? You're still working with almost baby hair at this age. It will be easier when she gets older. I agree that you can use cute head bands and stuff, and maybe braid it.
I would like to add that I once donated my hair to locks of love - the only thing I would say about that, is that they require a LOT of hair. O.O My hair was down to my butt and by the time they were finished with me, my hair sat just at my shoulders. It was cute but I looked like I was 12 lol. Like Trina said, they need 10 inches. And when you measure, do not measure from her head. Put it in a ponytail and measure from the hair tie down, cause they don't need your hair to be at least 10 inches long, they need to actually cut off 10 inches OF your hair length. But anyway, if you can get her hair that long and she wants to help the cause, that would be really sweet and I bet you could talk her into it.