Thank you for your query,
Your child seems to have Viral fever
In mysositis, the muscles will get inflammed and they will be tander to touch , associated with inability to walk and difficulty in walking.
The calf muscles are more frequently involved.
It can be diagnosed by a blood test-CPK NAC, which is elevated that the normal range.
Maintain proper hydration by giving plenty of oral fluids, Pedialyte
Proper bed rest
Monitor urine out put.
In most cases it is self limiting and will subside.
Hope I have answered your query.