"KInderen" is a "stacked plural" (if that word exists in English). The original plural of kind was "kinder" (just like in German). But when people did not recognize "kinder" as a plural anymore (especially in Holland) they added the normal -en ending to make it sound like a plural word. So in fact it is a double plural (or stacked plural). You could say that "kinderen" is just wrong in Dutch (double plural), but because everybody uses it, it is correct now. It is a bit like the word "museum". Because it is a latin word, its plural form can be "musea". But often people do not recognize this as a plural word, so they use "museas", which is stil considered wrong. But maybe in a hundred years time, "museas" will be an accepted plural form of museum. You should check with Duolingo then to see if I am right. ;-)
Plurals are easy. Plurals are always "de", regardless of what they take in the singular.
In the singular, it's not always easy tell the gender of a noun just by looking at it, so you need to memorize on a case-by-case basis whether it's a "de" noun or a "het" noun. There are some rules of thumb, though.