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According to Portugal's Immigration and Borders Service (SEF), it is not required when the child is travelling with one of his/her parents (added emphasis mine):
Minor, whose parents are married (to one another)
The travel permit must be granted and signed by one of the parents, only if the child is travelling without either of them; in cases where the child is travelling in the company of one parent, no permit is required, provided there is no opposition by the other parent.