You really don’t need to do anything special in terms of fueling for an 800m or 1600m race. Quite the opposite. The best thing to do is to eat and drink what you normally would during your hard workout days.
This might be difficult for you to consider, being a parent, but one possibility is that this is psychological, not physical. Before you rush to your keyboard to respond, I should add a caveat: there’s no way I could know this with certainty. I don’t know your daughter. But it’s something you should consider. Given what you said in your post, I think it’s the most probable explanation.
You state that your daughter had blurred vision, lightheadedness, couldn’t eat for days, after 800m and 1600m races. If she really does have a metabolic disorder, anemia, or something like that, she would have the same symptoms during training. There’s nothing special about a race versus intense training, metabolically speaking. And the metabolic demands of an 800m or 1600m are really not that great. These events are just a few minutes long. If you do a workout, say, where you warm up, do 8x400, and cool down, you are using more energy, net, than you do in an 800m or 1600m race. If you go for a 5 mile run, same. If your daughter is doing fine in training but tanks in races, then it’s probably not a metabolic issue.
By my lights, this is probably what’s going on: Running is really important to your daughter. You love your daughter, and you want to do everything you can to help her succeed. But you are doing too much. You are overbearing parents. You think that you are helping by analyzing every aspect of her training and diet, but you really are just making things worse. Your daughter feels an undue amount of pressure to succeed. She feels like a failure if she doesn’t win or set a PR. Your daughter has a bad race, and she feels like she let you and herself down, so she goes into hysterics, acting like she’s about to faint, telling you she has blurred vision, etc. This gives you, the overbearing parents, some kind of explanation for having a bad race, other than just simply not performing well. You probably have considered this, but you don’t want to admit that your daughter is making stuff up, so you go along with it.
Just relax. Having a bad race is not the end of the world. It's part of the sport.