"These are my children."
Translation:Das sind meine Kinder.
These is indeed "diese", but "Dies sind meine Kinder" is rarely used or maybe never while "Das sind meine Kinder" is the normal way to say it. So if theres not a special case, where it really matters to differenciate between this and that (dies und das), then you should stick with the more natural way to say it.
Sakasiru, one of Duo's "geniuses" wrote a very helpful explanation about the use of the demonstrative pronouns, which clarifies the usage of das, dies, diese, dieser, diesen, and more obscure ones. I confess that I wrote Sakasiru asking him to expand on plurals because I still don't know why "dies/das" are accepted and "diese" was rejected here. Sakasiru, himself, in his explanation has given as an example "These children are loud - diese Kinder sind laut. (nominative pl)" So, my guess is that Das/ Dies/ and Diese are correct and should be reported to Duo.
As for "meine kinder," note that in this sentence "kinder" is in the nominative case and is plural, therefore "meine." (all plural possessive pronouns, except in the dative case, follow the feminine declension. (Remember that so do the articles der-die-das, plural = die in the nominative and accusative.) https://deutsch.lingolia.com/en/grammar/pronouns/possessive-pronouns