I second the suggestion of a sports nutritionist. We have particular issues with our gymnast - lots of food intolerances and a food allergy. It was very difficult to figure out how to get all the nutrients she needed into her to keep up her energy levels for training. She'd get to the end of a training session and be absolutely wiped out, whereas gym mates who seem to eat air only would still be bouncing with energy!
I presented the situation to the sports nutritionist, and she agreed that it was a difficult situation and could see why I had been having trouble! However, she gave us great advice that has really helped. Things like never worry about fat content as she needs it, eat a second lunch before afternoon training, at least 5ograms of protein (in our case meat, due to dairy allergy) in every meal, a good range of fruit, veg and carbs.
She also advised that we should give her pure glucose tablets for training - she usually has one before and one or two during. The days she forgets or doesn't have enough with her, you can REALLY see the difference at the end of a training session - she melts out the door instead of bounding out.
Many of these things are things you wouldn't do with "normal", not quite so active children (and nor do we do them with our non-gymnast) but for her, they are an important part of keeping her happy and healthy, and on track with training. Her old head coach knew that we had some specific issues with her diet, and always was terribly supportive even though what we were feeding her seemed a little unusual at times.