Translation:The children that we have had are grown-up.
Well, it's a bit strange to see that someone's children should be referred to as somebody "we had", as if they were déjà morts or does anyone really believe that a person whose child has come of age has no child anymore? Like, 'Do you have any children, Mr Benson? - No, officer, my son George is already twenty-two.'
Oh G Benson's podcast is in my ears... No, I actually referred to a Mr Benson from the English Streamline course. And my lament goes to the other option, i.e. 'The children we had [...]'. Now, more generally, I find the whole English sentence suffering from some redundancy syndrome: why do we really have to say so about our children? Would it not suffice, in a day-to-day idiom, to call them just that, our children?! Otherwise, it sounds to me rather biblical: He hath known her and she hath had a child by him ))) Note that the car we've had is now out of whack is perceived differently.
Sorry Sitesurf, but you can't refer to a group of mixed-gender infants using the masculine gender. That is not allowed in English. You can use the pronouns "them" or "they," or you can use another generic word like "kids" or "children." If an acceptable translation uses "boys" in place of infants (though it shouldn't), then "girls" must be accepted also. We are apt to use man/he generically when we are referring to mankind.
Human beings create sentences, in every course. They work in teams (natives from both languages) to adjust translations. It is not an easy task because lessons are about grammar, syntax, conjugations, idioms, various registers of language and vocabulary is scarce, particularly in early lessons.
I don't know where you say the children we'd have grown-up as a correct translation, because it is not, and therefore it is not in the system. And in the system, contractions are not used.
There is no room for "would" here anyway, since the French sentence is in indicative compound past, but "the children we had have grown-up" is a correct solution.
Sitesurf, thanks for the reply. I agree, it doesn't make sense. - But it is definitely in the system. After I got the answer wrong. It displayed 2 possible translations. The 1st one was "The children we'd have grown-up." (with contraction) and the 2nd one read "The children that we have had are grown-up." I cut & pasted the translation from the screen to make sure I got it right.