It's genetics! For some people it is their genetic make-up and no matter their weight, health practices, or size, they are going to have it. My mother has always been athletic, svelt, and petite but the poor woman struggled her whole life with cellulite on her upper thighs. No matter how hard she worked out, no matter how grueling the cardio workout she did, no matter how strict her diet, those bumps and buldges wouldn't budge!
My grandmother made my mother an absolute nut about the cellulite. Neither my grandma nor I had it. So my very body concious grandmother totally had my mom in an absolute depression and traumatized about it. Which is ridiculous because she looks fabulous in clothes and swim suits. If she has a tan and lotions-up, the bumps aren't as noticeable...and to be honest...they're not even that bad. You have to be looking for them or she needs to be somewhere where the light casts shadows.
Nonetheless, mom had developed a serious eating disorder which periodically through the years she has battled...all because her mother was bothered by the cellulite on her legs. I guess the point I'm trying to make is, don't even bring it up to your daughter, let alone get her freaked out about it. It isn't any indicator that she's overweight, or going to be overweight. All you'll do is create unnecessary psychological turmoil for her. If she's not bringing it up...no need for you to do it for her!
If somewhere down the line *she* is really bothered about it, there are things she could do when she's older. Mineral water helps the system flush out cellulite naturally. A high fiber diet helps (lots of fruit and veggies) to flush the system as well. There are some spa treatments such as herbal wraps or a non-surgical process called Endermology, that uses a vacuume to smooths the skin's appearance. But to suggest she change her diet or get all into spa treatments now would be psychologically and physically dangerous and unnecessary. She is a child after all.