« Il est médecin. » translates to “He is a doctor.”
In French « Il est... » plus a noun does not use an article at all, so « le » is not used and « un » is not used. In English, we do need to put an article here, either the definite article “the” or one of the indefinite articles “a” or “an” for words that start with a vowel sound.
You have it backwards the definite article is not used in the French and I think that British English has borrowed that for this expression though usually when French indicates “Il est....” plus noun English usually puts an article. It is not wrong as “He is a father to six children.” It is not stilted or uncommon here in the US (I live in California.), though we also use “He is the father of six children.” If you use “He is father to six children.” without any article indicate where your dialect is from and report it as another alternative to Duolingo.
It is not usually accepted for children, but since some people requested that it be accepted at a sentence by sentence level, it is possible. Since then, Duolingo added “gamin”/“gamine” which are each specifically translated to “kid”, so now they will expect “enfants” to be translated as “children” and the exercises that allowed “kids” for “children” can be reported so they can eliminate that discrepancy. The plurals “gamins” or “gamines” will translate to “kids.”