Just to clarify for those who've replied (thanks for replying btw). I wasn't stating that it is grammatically incorrect, but that it just didn't sound natural to me. Yoda, for example, tends to speak within grammatical rules, but uses unnatural structure (per current usage at least). The main thing is that my version should also be an acceptable answer and not counted as incorrect. After all, this is for learning Czech, not English. I think that these should be considered acceptable answers as they all capture the sentence's meaning in my opinion: "Zofie was a mother of ten children" "Zofie was a mother to ten children" "Zofie was the mother of ten children" ...and probably others.
OK, but I don't see your reason for your preference. Grammatically correct doesn't mean mandated. For me, I am leaving the real-life conversation open, because there are obvious comments to make about the mother's achievement. (Btw, I am hearing echoes in my head over the years of slav migrants speaking english and leaving out articles. I am enjoying learning some Czech.)
I agree that "a mother of ten children" sounds unnatural. The ten children have exactly one mother; thus only one mother can be the mother to those children (barring a very unusual situation), and so only "the mother of ten children" is correct. "Mother to ten children" could also be acceptable, because that would imply that "mother" is referring to a kind of profession rather than a person, but "a mother of ten children" is at best not the most natural choice.
Actually, we have discussed here before in https://forum.duolingo.com/comment/26460699/Je-to-matka-%C5%A1est-set-%C5%A1edes%C3%A1ti-dvou-mal%C3%BDch-pavouk%C5%AF
For you this may sound simple and clear but I can assure you that this particular usage is very confusing for an ESL speaker like me and it does not follow the usual textbook rules. Basically, I have to remember this as a weird exception.
And I did not create this sentence, so I am not alone. Anyway, "the mother" is accepted and has always been. We may consider to change the defaults in these sentences, but I will leave that to those who know this grammar better.