The word Mädchen could mean both, singular and plural. So, to distiguish between the two, you have to look at the context. The verb "essen" is used for plural subjects (we, they) and the formal "you". So here, we know that Mädchen means plural : the girls.
In addition, as Christian said, if there's no article used before the word "Mädchen", it means plural. (http://duolingo.com/#/comment/159264) I've just known it though.. :D
- Das Mädchen = the girl
- Die Mädchen = the girls
- Mädchen = girls.