This thread is pretty old. But I'll throw my .2cents in anyways.
At 10 years old. The best bet for your daughter would be to find other physical activities that she enjoys. You want your daughter to be well rounded. So if she was involved in other sports that test
her physically in ways that differ from soccer would be best. If she was my daughter, I would recommend basketball. She could use the hand eye coordination training that basketball provides, as well as the sprint and vertical leap work. Basketball is a winter sport as well and shouldn't interfere with soccer in the summer.
Sprints on the treadmill get boring pretty fast, she would have to be pretty dedicated to put up with that. Sprinting up and down a gym floor, dribbling a basketball - scoring lay-ups at both ends = fun for a 10 year old.