Does anyone know which law would apply? And does anyone know about this claim of 25-30% of my income? And if Brazilian law would apply, do they have any authority to force me?
The law (brazilian) merely provides that both parties are supposed to provide in the upbringing of the child equally. It does NOT provide a figure of 30%. However, according to a decision by the supreme court, the party that pays alimony may never pay more than 30% of his income. This 30% is usually only applied in cases where there are several children.
In principle they would have the means to force you due to a treaty. Obviously they could also put you in prison if you arrive here to visit the child.