Tazdev, (putting on my cardiac nurse hat), there are two types of cholesterol - that whiich we eat, and that which our bodies make. The latter tends to run in families. However, sometimes an analysis of a child's diet reveals that the child is inadvertently eating a lot, even when that child is seemingly healthy, normal weight and active. This happened to my sister's daughter when she was giving her a lot of cheese, thinking it was "healthy". Many kids also eat things like mac n cheese, hot dogs, chicken fingers and fries, grilled cheese, etc, regularly, and that would need to change if it were the case. Just like anything else, a varied diet of vegetables and fruits, whole grains, lean meats, limited snacks, and a handful of nuts daily (which have good cholesterol) will help lower cholesterol naturally. Oatmeal helps, too. And exercise raises good cholesterol. The danger of having fat circulating in the blood is that it builds up in the arteries, especially the small ones that are in the heart and brain, etc. A young child still has growing to do, so a "diet" isn't really necessary, just maybe a change toward healthier eating and exercise.
I think I've said here before that they looked at the coronary arteries of young American soldiers in the Vietnam war and found that even at a young age, they had the beginnings of artherosclerosis, as opposed to their Asian counterparts, who did not. This they chalked up to the American diet. And certainly it's gotten worse since then. As above, it would be great to have her see a nutritionist/registered dietician, which I'm sure they will as part of a workup at CH. I took my kids when they were younger myself. It was kind of comical because insurance wouldn't pay for it because they had no identified "need", but I wanted them to go anyway and I said I'd pay for it. (Having my mother living with us was a challenge as she loves feeding "comfort foods", snacks and candy to kids; we had a lot of arguments about it.) I wanted them to hear from someone besides me, i.e. an "authority", about proper things to eat and proper serving sizes, etc. It was an eye opener for them. She had a test tube of salt and fat in there that was labeled "hot dog", because, essentially, that's what a hot dog is. She also said that things like chocolate and hot dogs are "vacation foods", cheese on a sandwich should be no more than once a week, a serving of rice was 1/3 of a cup, etc. That was years ago but those lessons have stuck.
DS is an athlete with an athletic build, he can eat what he wants, but I still stress to him that for heart health, he needs to watch the fast food and junk (a staple of his and his baseball buddies in season; a ritual, really). DD was always good until last year when she gained a little working in a restaurant - snacking there became the norm and she gained some weight, plus I think she had some emotional eating going on as well. But she got a hold of it this summer on her own (well, with a little help from me, mainly providing healthy foods and giving her support and resources) and she managed to drop about 15 lbs on her own. She also does yoga, some weight training, rides her bike and we walk together.
HTH. In my experience counseling people on diet (in the hospital after a cardiac event), most are either uneducated about what their diet should be (as most of us are in 2015 - everything is super sized now; we need to go back to when it wasn't like that) or are in denial about it. It is hard to get through, but we do have a nutritional staff who work with patients on that alone, as well.