If she can eat at least an hour before class that would be best.
Muscle weakness showing up in a pointe class might mean that she needs some extra exercises every day to build more strength. Unfortunately I cannot see her dance and explain that it is her feet, her legs, or her back that needs strengthening. I would ask her teacher. She would be the one to know what area needs daily work to be strengthened.
I think you are right to discourage the eating of junk food. Anytime! Snacks such as walnuts, cheese, celery, a good quality beef jerky, fresh fruit, will ALWAYS produce more energy than junk food. In fact, junk food, with its added sugars and starchy content is a tiring kind of food. It takes a lot of energy to digest and gives out little. Unfortunately the food issue comes up a lot as a disagreement between parents and children.
The only other thing I can suggest is that she do foot muscle exercises. Again, I would ask her teacher, but if your daughter would not want you to, then she should ask the teacher herself, what exactly would help her strengthen faster.
I think it is important to stay hydrated and maybe have a very light but nutritious snack beforehand. I did dancing as a child and my mom used to insist on heavy meals beforehand because she obviously meant well but did not know better. For me it was absolutely unpleasant and made me feel sick during my dancing lessons. Let your child develop a feeling for when to eat. And avoid heavy meals prior to dance lessons. That would be best.
It is better to leave the body time to digest: thus, to prevent digestion disturbing the session, it is recommended to wait at least 2 or 3 hours after a meal before starting any physical exercise. The body needs to assimilate nutrients before being put to the test.