Travelling with kids
innsent av Rene Limburg i Brasil forum
Even travelling with my wife's own kid and with the documents signed by the father (me) my Brazilian wife was not allowed to leave Brazil.
You have no chance.
Normally you would have a document signed by the Mum to say that the Dad is the sole guardian, but if you cannot do that you are in a difficult situation. If you are determined then I suggest you consult a lawyer. Perhaps you can become the official guardian. That would help.