Diet has to be the first place to look. What is she eating? At her age you need to be the boss of what goes in her mouth. learn early is the way to go. I don't think trainers and gyms are needed for kids. What kids need is to play. Like proper play. Outdoors, up trees, on bikes......all that stuff we know to be good for them. Much for fun for her to go for a bike ride than to walk on a treadmill. My fear would be that she would resent the treadmill, and it could impact negatively on her self image. Wii fit? Play together. So many options to do that are low to no cost, walking as a family, swimming, playing ball, etc.
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