Hi, "Kindern" is the dative of "Kinder" - they are both plural. Das sind die Kinder. (Nominative) Those are the children. Ich komme wegen der Kinder. (Genitive) I come because of the children. Mein Sohn spielt mit den Kindern. (Dative) My son is playing with the children. Das Geschenk ist für die Kinder. (Akkusative) The present is for the children. Which case is used depends on prepositions and verbs used...this might help: http://www.vistawide.com/german/grammar/german_cases_dative.htm :)

Sabine is absolutely right. In addition, this occurs with all nouns which can add the 'n' in the dative plural. Some can't for phonetic reasons, e.g. because they have already an 'n' in the plural, die Frauen (nom.) - den Frauen (dat.), others like die Autos - den Autos.

But if you start with the grammar you will get crazy over German, I watched a lot of friends struggling with it and finally decided to start this channel: https://www.youtube.com/channel/UC2XoO1HBizGfvZR01kK_VJQ you listen, read the subs and understand more and more. Good luck!