Thank you for your query on Healthcare Magic.
Your child seems to have a viral fever
The legs pain is due to involvement of muscle by the virus called myositis
It can be diagnosed by doing blood test
for CPK levels.
I advise you to give Tylenol in prescribed dose for pains.
Bed rest is advised.
Give plenty of oral fluids to maintain hydration.
Consult your Pediatrician
for the examination of your child and to rule out Lyme diseas by doing appropriate investigations.
Hope I have answered your query.