Learn Dutch: Onze kinderen spelen met de kalveren. - Our children play with the calves.

A small group of neuter (het) words take this irregular -eren plural. kind becomes kinderen (children) and kalf becomes kalveren (calves). There are only about a dozen of these in Dutch, so it is best to just memorize them when you see them!

Why does the f in kalf change to a v in kalveren?

In Dutch, when a word ends in f, it often changes to a v when you add a plural ending. Because the -eren suffix starts with a vowel, the harsh f sound softens into a v, making kalf turn into kalveren.

If kind is a het word, why does the sentence use onze kinderen instead of ons kinderen?

You would indeed say ons kind for a single child because it is a het word. However, in the plural, all nouns act like de words (de kinderen). Because plural nouns always take de, the word for our must get an -e at the end, becoming onze.