How to deal bullies that prey on younger children?

I have a younger cousin who's barely 5 being bullied by a bunch of 8-14 year olds living in the same apt complex, and one do*chebag pushing 20? Everytime I visit, I would hear them running all over the place , mocking and mimicking my cousins manner of speech and actions ( to the pnt where it gets prtty obvious already). On one occasion they did this in front of her mom etc. The thing is, my cousin is starting to get a little bothered by it.. SHe doesn't have troubles at school, infact she loves going there bec that's the one of the few places she actually feels comfortable in ( friends, teachers, etc).. but once she gets home.. I mean I can tell it's bothering her. The kids are pretty loud so even if the windows are closed I can still hear them. Their parents don't mind, so talking to them is a lost cause ( there's been some minor altercations , not physical , between them and her mom). Her mother is too busy with school and work , so talking to her about more problems gets difficult. I don't know what to tell her? When I visit, I usually just take her out to the park/movies/shopping malls/museums/ etc but that gets tiring AND expensive... What pisses me off more is the fact that they are bullying a child who's so much younger than they are? Aren't they supposed to be doing something productive with their time? or pick on kids their own age level? I've given her some origami books to divert attention but I don't think its working. I don't what her self esteem to be diminished... or should I just let it be and let her sort it out, fight her own battles? srry for bad english, not first language!!