Thank you for your query,
Your child had suffered Viral fever
In myositis, usually the leg muscles will get inflammed and present with pain and inability to walk with pain.
In this condition CPK-NAC will be raise and suggestive of muscle inflammation
Proper rest, paracetamol
for fever and pains, maintenance of hydration by giving plenty of oral fluids are required with monitoring of adequate urine output.
Hope i have answered your query.