Parents should be charged for doing what they believe is right for their child even if it goes against the law because laws are the conglomeration of standards that society must follow to maintain civility. We may not always like the law, but the way to deal with that is elect legislators who will change the law - not take it in your own hands.
While one might be tempted to say no, simply because this case, there are far worse cases of parents mistreating their children. We have all heard about parents that try to treat their kids cancer with prayer and crap like that. I think that IF a parent ignores competent medical advice in favor of crystals a vodoo woo woo, then they should be charged with child endangerment.
However, if the parent listens to sound medical advice, and also looks for holistic approaches for use in conjunction with actual medical treatment, then I don't see a problem.
First off, I'm not sure what type of child illness would require the use of marijuana to treat. This seems to be a case of negligence on the parent's part. Whether the parent is using marijuana medicinally to treat their own ailment or not, a parent shouldn't use an illegal substance but rather consult a physician.
Ultimately, a mother knows what is best for her child because no one knows the child better. This mother in particular had no success in relieving her daughter's symptoms and was forced to watch her child suffer on a daily basis. She did the research and risked so much simply to give her daughter some relief. There are so many arbitrary laws that don't take into account how they affect our daily lives and it is much more complicated than the black and white way they are presented.
There are extreme cases where the government can intervene, but in this case, where her actions were supported by science, even if they are not supported by the law, she should not be persecuted. The girl had a very serious case of epilipsy and nothing else worked. Marijuana should be legal.