I've gotta say, bmi is a load of rubbish for alot of people, for example, my son, he's classed as borderline overweight for his age, he's 8, and in age 7 clothes and they drown him, there nothing of him at all, if anything he looks like he's not been fed for a month as he's so active, he doesn't know how to sit still lol. My bmi at last check was 30.5, apparently obease, but at a small size 14 at that check and my midwife saying that I hold my weight well and don't look the "norm" for my bmi to be that high, and other health related factors that were checked at my booking appointment and since she has commented about being extremely good for someone of my bmi, I so you can have a high bmi and still be healthy. My partner also, average height, not much of him, but a bit of muscle as he has a very physical job, and he's classed as borderline overweight. It really doesn't take into account muscle mass, bone density and other factors.
Just because she has a bit of a belly doesn't mean she's an unhealthy weight. You're looking at it as body shape, which she might pick up on and then end up with body confidence issues later in life, the average person will not look like a swimsuit model, ever, that shape does not suit everyone. I have a friend, she's 5'8" and when she was the top end of the "ideal weight" for her height it made her look anorexic and very ill, she gained a couple of dress sizes, which then put her in the nearly obease section of the bmi chart, but she had a really nice body, which suited her.
Your dd has to find a shape she feels most comfortable in as well, don't pressure her into looking a certain way, she will pick up on it.
By all means help with getting healthier options in her normal diet, as this will help keep her healthy in the long run, and involve her in meal prep, and make it fun for all of you, but don't pressure her into anything.