Mädchen is a neuter noun in the singular, but becomes feminine to indicate plurality. Because this is in the genitive case, the "die" becomes "der," and so you can infer that Mädchen is plural in this context.
In other words, since Mädchen is not masculine, it could never be "der Mådchen" and be singular, because the gender wouldn't agree.
- I am not a native speaker and am just trying to explain my reasoning for why I think this makes sense.
I know why it's that way: it's because both feminine and plural become "der" in genitive: -Das Kleid der Frauen / The dress of the women. -Die Autos der Mädchen / The cars of the girls. -Die Handtaschen der Damen sind rot / The ladies' handbags are red. As well as: -Das Haus des Mannes / The house of the man. -Die Hunde des Jungen sind gefährlich / The boy's dogs are dangerous.
It is "der" because that is the genitive article for the plural "die". "wegen" takes the genitive, so in this case, because it is "because of the girls", you use "der Mädchen". If it was "because of the girl" (singular), it would be "wegen des Mädchens", since the singular "Mädchen" is a neuter noun (because of its diminutive suffix). Check out this article: http://german.about.com/library/blcase_gen.htm
No, "deren" means "of which/whose" and is typically used in conjunction with demonstrative pronouns like "these" or "those". Since there is already a noun here, namely "Mädchen", you just use the proper genitive article with it. The genitive for "die" is "der", so you just refer to them as "der Mädchen".
The table on this page is useful and complete, though it lacks some detail about exactly when to use things: http://german.about.com/library/blcase_sum2.htm