Die Jacken von den Mädchen sind da
I dont quite understand something here - this is a genitive case (whose jackets are those) so how come it is not "der Maedchen"?
Though in English you apply Genitive, in German, there are two ways of denoting possession: direct Genitive (the girls' Jackets) and using von (the Jackets of the girls). Von however ALWAYS takes dative; hence it is den Mädchen (dative-plural).
Using genitive, the sentence would be:
"Die Jacken der Mädchen sind da."