My sten son has a Spanish citizenship and I'm moving to Spain with a work visa I'd like to know if my wife can apply for a visa in Spain based on the premise that her kid has dual citizenship (Brazil and Spain).
As mentioned in the comments, the Zambrano case from the EUCJ is somewhat relevant but there are some important differences (in Zambrano, the child was already present in Belgium and schooled there) and it would seem difficult to use in practice. If you were already living in Spain and faced with deportation or somehow trying to legalize your situation it could be another matter but that's not a situation you want to be in.
Generally speaking, there is no straightfoward way for your wife to use her son's EU citizenship to move to the EU and in principle EU law is less relevant for Spanish citizens in Spain. There might be some relevant rules under Spanish law but I doubt it.
But your last edit implies that you intend to get a work permit for yourself independently in any case. It would seem easier to use that, rather than your kid's citizenship, to secure a visa or residence permit for your wife.
In European countries, it's usually possible to apply for a work permit for you and a visa for your spouse at the same time (I don't know precisely about Spain but I do know about some exceptions in other countries). If that's not possible, it should at least be possible to reunify your family later on (this leaflet from the Spanish ministry of Work and Social Security indicates that you can only do it after residing in Spain for one year).
Hopefully someone else can provide more details on Spanish law and practices in this area but it seems that the best would be to get advice from a legal professional, certainly if you are contemplating using a complex route like the Zambrano ruling.