"Mit den Jungen isst das Mädchen."
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.