"The children have the wine."
Den in this case is the accusative for Der.
Nominativ, used for subject or after verbs like "sein" or "heißen" DER-DIE-DAS-DIE
Der Mann ist alt
Die Frau ist schön
Das Kind ist neugierig
Die Häuser sind hoch
Akkusativ, used for direct objects DEN-DIE-DAS-DIE
Ich habe den Mann kennengelernt
Du hast die Frau gesehen
Ich kenne das Kind
Ich sehe die Häuser
When you will grasp it, you can ask me regarding Dativ and Genitiv cases. I am not native, but had to study it. :)
Accusative is the direct object of the verb - so, if it answers the direct question of "what", it is accusative - So the children have - "what?" = the wine - therefore, accusative case - the masculine gender noun is the one you have to pay attention to as it changes from its nominative or subject form.