The income would just be what has been earned either by direct employment or self employment (Plus any savings interest!) Tax Credits is not a taxable benefit neither is it employment income, it is not taken into account in PAYE therefore it would not need to be included on a Self assessment
I believe it would Be correct to just put the 20,000 income.
Although I have filled in this forms for my family members, I am not yet up and practicing so someone please feel free to correct me if I'm wrong :)