Motive behind kidnapping homeless kids in Gotham
In Gotham Season 1 Episode 2: "Selina Kyle", Patti tried two times to kidnapped homeless kids and one of them is Selina Kyle and in the end of this episode Officer Gordon and Harvey Bullock are able to capture Patti and she is killed in the police encounter.
But why was she doing that, why was she kidnapping homeless kids? The motive behind it is not mentioned in that episode or latter. Is this plot just created so that at last Gordon and Selina will meet and in that case the motive is just a secondary thing?
Because she was subordinate to the Dollmaker and abducted homeless kids for his experiments. She even mentioned that few times:
This wikia has a good description of her character. And why Dollmaker need those kids will be explored in later episodes or you can read above linked Dollmaker wikia page.