If the sponsor (co-sponsor) is self-employed, owns his own company, or if the sponsor (co-sponsor) does not work, then the employment contract and P60 can be substituted with certified copies of the previous two years' tax returns accompanied by a notarized version of his business accounts.
as a guidline you need to show that you have left, after rent/mortgage, council tax and loan repayments have been deducted:
105.95 per week for a couple
62.33 per week for each child
you can include any income you receive from benefits such as tax credits, child benefit etc.
but this is only a guide, probably get away with a bit less..
you need to show your wife has 'sole responsibility' and sole custody for the child.