Hi!
The only way to diagnose a congenital myopathy is with a muscular biopsy and a DNA analysis. If these have not been done yet, your child is just diagnosed as low tone.
Congenital myopathy is a muscular disease located on the muscle's fibers, and is not evolutive. It means that some muscles are weak and will remain so all her life. They can't be strengthened and there is no known cure for it. The treatment will consist in maintaining the trophicity of these muscles (hot/cold bath to increase the blood flood, massages), and their length as they will tend to tighten.
Low tone would benefit of a treatment including sensory integration and selective strenghtening, and neurodevelopmental techniques.
For these reasons, it is very important that you get a definitive diagnosis for your child.
At this stage what I would suggest for your child would be:
1 Baby massage
2 A pair of little boots with a stiff counter to help her stabilize her feet/ankle while standing
3 High kneeling with or without support, walking on knees
4 Pool (the water must be at least 30° or use a wet suit), swimming will help the blood flood, coordination and strenghtening if possible, use an appropriate support.

All these will help your child to progress while awaiting for the diagnosis, whatever it is.