Brief Answer:
Joint and muscle aches are common in flu, Naproxen should help

Detailed Answer:
Hi
Thanks for asking on this platform

I have gone through the query and do understand your concern.
Joint ache is very common in flu and causes distress and difficulty in walking. It is a part of viral infection and most of the time responds well to ibuprofen(advil) and tylenol(paracetamol).
But as you said that both of these are not much effective, you can try naproxen(also an Non steroidal anti Inflammatory drug like ibuprofen but more potent). Aspirin is also a good choice but should be avoided in children with flu as this may cause Ryes syndrome.
In most of the cases joint pain improves along with other symptoms but if pain persists longer please evaluate a pediatrician for clinical evaluation and to rule out other causes.

I hope this helps.
Please let me know if you have any more questions.
Regards

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