why the little girls reject my help

several possibilities - they may be too warned from their parents not to talk or approach strangers...
it also can be that you simply misunderstood situation - maybe it was not real bullying (boys and girls can show emotions in strange ways).
Actually, I think it was not real bullying, because if it was you would probably help them without asking.

Also, kids sometimes tend to deal with things between them. I don't know how is in China but in Croatia most of kids are like that. If I was bullied as a kid, bullied for real, and someone asked me if I need help I would say yes. It never happened though.

But if it was just teasing or something that would maybe upset me a bit, but nothing I thought as a real danger I would decline any help from adult. Like all my friends as well.

I think this a good analysis of the situation. It certainly wasn't anything you did, and you clearly meant well, but I don't really see the point in dwelling on it.

Perhaps they thought you were a molester. Children are taught from a young age not to talk with strangers. They could be abducted or raped. Simply for safety reasons.