Then the sons are free means they are exempt from taxes. Moffatt translates “So their own people are exempt,” and New English Bible is similar; Good News Translation has “… that means that the citizens don’t have to pay.” To retain free some translators have said “Then the citizens are free from having to pay tax.”

Quoted with permission from Newman, Barclay M. and Stine, Philip C. A Handbook on the Gospel of Matthew. (UBS Handbook Series). New York: UBS, 1988. For this and other handbooks for translators see here .