Translation:The senior citizens are playing with the children.
No, it's not weird. Think of it this way:
You live in a German city, where there is an music academy and orchestra. The orchestra has 3 different divisions (age groups): youth orchestra, regular adult orchestra, and a special one for senior citizens who love music. Each month, 2 of the divisions are selected to play a concert together. You see that on the online schedule for this month, the youth and senior orchestras will be playing together. You decide to call and invite your friend to the concert. Your friend asks you which division will be playing together, and you might reply "Die Senioren spielen mit den Kindern."
my favorite quick reference for the definite article cases is http://upload.wikimedia.org/wikipedia/commons/c/cb/German_definite_article_declension.png
and for indefinite article cases is http://upload.wikimedia.org/wikipedia/commons/thumb/5/5d/German_indefinite_article_declension.png/350px-German_indefinite_article_declension.png