Learn German: Ich kenne das Kind, und ich sehe es oft. - I know the child, and I see it often.

es is the accusative personal pronoun for "it". Here, it replaces das Kind (the child). Because Kind is a neuter noun (it uses das), we must use the neuter pronoun es to refer back to it.

Is das Kind in the nominative or accusative case here?

It is in the accusative case because the child is the direct object (the person being known). You might not notice the case change because the neuter article das looks exactly the same in both the nominative and the accusative!

I learned that "to know" is wissen. Why are we using kenne here?

German has two common verbs for "to know". You use kennen (used here as ich kenne) when you are familiar with a person, place, or thing. You use wissen when you know a fact or a piece of information.

Why does oft come at the very end of the sentence?

In German, adverbs of time or frequency (like oft, meaning "often") usually come after the verb and after a direct object pronoun. Because es is a pronoun, oft gets pushed behind it.