Learn Dutch: Het kind is klein. - The child is small.

Dutch has two words for "the": de and het. Every noun uses one or the other. Kind happens to be a "het-word" (a neuter noun), so it takes het in the singular. You just have to memorize the article when you learn the noun!

Why doesn't the adjective klein have an -e at the end?

Because klein comes after the verb is instead of directly in front of the noun. In Dutch, when an adjective describes a noun but comes later in the sentence, it never gets an extra -e. If you put it right before the noun, it would be het kleine kind.

How would I make this plural to say "The children are small"?

You would say De kinderen zijn klein. The plural of kind is irregular: kinderen. Notice that the article changes to de, because all plural nouns use de for "the", even if they are het words in the singular!

What is the infinitive (dictionary form) of the verb is?

It comes from the verb zijn (to be). Just like the English verb "to be", it is highly irregular. Because het kind is a singular subject (like he, she, or it), we use the form is.