I have never heard of any doctors prescribing oxcarbazepine for myotonia, but it is in the same class as carbamazepine and lamotrigene, so it makes sense. It does have some side effects like low serum sodium and sun sensitivity that might make it less desirable unless there is also a mood disorder that needs treatment. I am always concerned about the effect of anti-epileptics on personality, but since she has taken it since she was an infant, hopefully it won't be an issue...it is most critical to keep an eye on it during teen years and early adulthood when hormones can accentuate mood changes. When she gets older you may want to experiment with Mexitil to see if it gives her more relief.
Have you done DNA testing to see whether it is a sodium or chloride ion channel myotonia? There are dominantly inherited mutations in both.
It is really tough for kids with MC in school. We all have horror stories, but we survived somehow :-) The biggest challenge for most of us was PE class. You can imagine the terror at having to do something like a gymnastics rotation with myotonia.
You might want to check into MDA camp for Delaney next year. It's for ages 6-17. My two granddaughters have gone for many years and love it. They'll meet other kids with myotonia, but they will also see children with much more serious muscle diseases and it kind of puts things in perspective.
Since she has some pretty serious stiffness and cramping, she may do well with a little magnesium every day. You can even get a topical oil that seems to work well - you just spray it on. It's important to keep an eye on her serum sodium...if it drops she will probably have worsened myotonia. Sodium and potassium are included in metabolic panels.