I will try my best to answer your query.
Voice changes sometimes follow an upper respiratory infection
lasting up to two weeks. Typically the upper respiratory infection or cold
causes swelling of the vocal cords and changes their vibration
resulting in an abnormal voice.
(voice rest) typically improves the voice after an upper respiratory infection, cold, or bronchitis
I advice you to treat the cause that is upper respiratory tract
Meet the child specialist they will prescribe you the right drug.
Hope my answer will be helpful for you.