extension of stay in L visa

my son was granted with 14 days stay single entry L type, he's only four years old and travelling with me for the first time here in Suzhou. I was granted double entry with 30 days stay each. I want to extend my sons visa for 12 more day. we plan to stay in china for 26 days. When should I go to PSB for filing of extension and how much money is needed?