Translation:My children's teacher does not eat fish.
The teacher of my son = My son's teacher The teacher of my sonS = My sons' teacher
Hijos = "sons" or "children" (which would be the better translation, since we do not know if all the children are boys)
The apostrophe should be at the end of the word "sons" if it is plural and possessive. Duolingo was right in this case if you wrote "son's."