First and foremost, you need to tell your granddaughter that it is okay to fail as long as she keeps on trying, and that every failure gets her a step closer to getting it right. After all, Math is indeed a tough subject.
I have consulted my sister regarding your concern. She's a teacher by the way, and has had a lot of experience in teaching kids. She told me that whenever a child is having a hard time in a particular lesson, the usual approach is to make them take remedial classes because this is where they will be taught hands-on. Remedial classes are very different from the regular sessions because activities were done instead of the usual lecture type. Also, since it is mostly a one-on-one class, a child has more freedom to ask the questions on topics where he or she needs clarification. Typically, when a child does not fully understand a lesson, oftentimes they are afraid or too shy to raise their questions in front of the whole class. Hence, remedial classes provide them the venue to talk with their teachers about even the slightest detail that they need to be enlightened.
I also think that no one knows the performance and the needs of a child better than their teachers. So if you think you can't help her because you're afraid that you might not be able to teach her what she needs, then allow her teachers to do it.