"My baby plays with the children."
Translation:Mein Baby spielt mit den Kindern.
After the preposition "mit", you have to use the dative case. In the dative plural, all nouns add an "-n" or "-en". The only exception are nouns whose nominative plural already ends in "-n" (e.g. Frauen) and nouns whose nominative plural ends in "-s" (e.g. Autos).
"die Kinder" (the children, nominative plural) becomes "mit den Kindern" (with the children, dative plural)