Her mother and grandparents only feeds her junk food and she has gone from average weight to top 99% over the last 3 years.

Three years ago her mom's parents retired and offered to watch the her after school rather than having to continue paying for daycare. It was a huge financial relief so I accepted. Since then after school she just lays around watching TV and eating junk food rather than running around playing like she did at daycare. I immediately noticed rapid weight gain and more fights at dinnertime and discovered that the "snacks" she got after school included things like full on adult fast food meals or things like ice cream and brownies on a daily basis. In this time she has grown from average size to morbidly obese and in the top 99% weight for her age according to her annual growth charts. I explained to them that my family has a history of diabetes and blood disease and that I'm concerned about her health if this continues and to please stick to healthy snacks like fruit, vegetables, and maybe thinks like sugar free yogurt or pudding as an occasional treat, so that she wouldn't continue the rapid weight gain and would still have an appetite at dinner time. This lasted a couple of months but they're right back to it after repeated conversation and now their "healthy snacks" are things like granola bars, fruit snacks, full sugar fruit juice, and they're still giving her the other things but just not telling me about it when I ask what she had (she mentions it regularly).

I'm planning on looking into alternative after school care for the weeks she is with me and talking to a doctor about getting a written dietary plan to try to get the weight gain under control. Is there anything legally I can do to help make sure they take this seriously and aren't just brushing aside the things I'm trying to do to get my kid to eat better and stay healthy?