converting s2 to s1 in Shanghai

I work in China and my children (8,3,4) moved to China a few months ago. Because my Z visa was expiring in less than 6 months, my kids got an S2. Now Shanghai PSB refuses to extend it where as I am working legally and paying all my taxes. They are asking my kids to go back to Pakistan and get s1. Where as last year my eldest son got s2 converted to s1 in Nanjing. How can I overcome this? I dont want to send them back and I cant afford to send back 3 kids.