She is 16, does about 14-16 hours of exercise (dance - Ballet et al) a week
She is 5ft 2/3 (I haven't measured but I'm 5ft 5) & weighs 7stone 7lbs; she eats loads (mainly carbs - I think she needs more fat & protein)
But she looks skinny & her muscles stand out
I worry - It's my role in life 
BMI doesn't really work as it doesn't take into account muscle/fat (I understand)