Thank you for your question.
You do not mention the age of your child. Muscle pains with a flu-like illness
is not uncommon. It's caused by inflammation of the muscles and is part of the general inflammatory response that occurs with flu.
It is likely to pass in a few days. In the meantime, I advise you make sure your child has plenty to drink and if the pain is troublesome, paracetamol
I hope this has helped you.