Magnesium can help as long as you don't use too much (it will increase potassium levels which in turn can worsen myotonia). It would take quite a bit to do that, so I would just work up gradually and stop at the dosage where you see relief.
I would suggest magnesium oil so that your daughter doesn't have to swallow a pill. I've gotten very good feedback. It may burn just a little - some people say they spray it on the bottom of their feet to avoid that. This is a brand that has very good reviews: