I'm not sure where the problem is. Here's the complete table, singular/plural:
Nom: das Mädchen / die Mädchen
Gen: des Mädchens / der Mädchen
Dat: dem Mädchen / den Mädchen
Akk: das Mädchen / die Mädchen
There's plenty of tables like this on the Internet.
"Von" takes the dative, so "von den Mädchen" is correct. "Des Mädchens" is also correct.
There are several ways to create nouns from verbs, the simplest one is to take the infinitive and use it as a neutral noun:
Nom: das Gehen
Gen: des Gehens
Dat: dem Gehen
Akk: das Gehen
Again, this just follows the table for the neutral nouns, and you can find plenty of them on the Internet. There's no plural, though, no matter which case, just as there are some other nouns which are not derived from verbs that don't have a plural. You have to switch to words like "der Gehvorgang / die Gehvorgänge" or similar to get a plural.