"Die Jungen essen mit den Mädchen."
One of the English translations offered is a bit questionable... or maybe I've been on the internet too long again.
Plural: mit den Mädchen = with the girls
Singular: mit dem Mädchen = with the girl
It's "dem", not "der" because surprising as it may seem, "Mädchen" is a neuter (!) noun.
Yes. "Die Jungen" (the boys) is in fact the standard term. "Die Jungs" (the boys) is used colloquially in some regions, especially in Northern and Central Germany.
slight audible difference between dem and den, once Mädchen is the same for plural and singular..