"how long has you child had a fever for?" or "how long has you child had a fever?" are what an English person would ask. "Since when has your child had a fever?" would only be said by a non native speaker.
In British English 'since when ' indicates scepticism. E.G. 'since when have you been king of France? '
I agree. This is a poor English translation. A doctor would ask ' how long has your child had a fever' or 'when did the fever start'
since when does your child have a fever - mais pourquoi pas accepté ?
'Since when has your child had a fever' would be OK. 'Since when does...' sounds odd.
How long has your child had a fever? Not accepted 19/07/2020
This does sound much more natural to me.
Since when your child has had a fever?