"Mit den Jungen isst das Mädchen."

[Aitchy]

Why is this not, "The boys eat with the girl?"

[GregTrotter]

Because of the cases. It's "mit den Jungen", not "mit dem Mädchen".
If it was the other way around like you suggested, the sentence would be:
Mit dem Mädchen isst die Jungen" Although I am not positive if I spelled jungen write in this case...

[Menschenkind]

You're right. But in your last translation, it has to be 'essen die Jungen', since they are 3rd p. pl. :)
To make this sentence much clearer, one would usually say: Das Mädchen isst mit den Jungen. 'the boys eat with the girl' would then be: Die Jungen essen mit dem Mädchen.

[GregTrotter]

Of course. Stupid me to forget that!

[Menschenkind]

Apart from the wrong verb, your explanation was fine. Don't worry.