In German: das Kind --> die Kinder. In Dutch: het kind --> de kinderen. Why this termination «en»?
"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. ;-)
That is exactly how the word evolved in English. Singular "cild", plural "cildru". Over time (and in parallel with ox-->oxen) the plural became "children".
The word "cactus" is like that.
It is the singular and plural form and people hardly recognize it as plural anymore so you hear "cacti" or "cactuses"
That's because the word comes from Greek (although some mistakenly think it comes from Latin, hence "cacti") and once a word enters the English language, it gets pluralized the English way, hence "cactuses".
Should it be pronounced like 'kindren' or like 'kindere' - and are any other forms possible (like 'kindre')?
I made a mistake and noticed that you could say " De Kids"… I didn't know you could write kids instead of kinderen!
Kids is used in Dutch as well. It doesn't have a singular form though. You can't say kid instead of kind in Dutch, only kids instead of kinderen.