Roughly... take off your personal allowance (single, married etc), take off a quarter for tax/NI (we all work a week per month for nothing!)... devide what's left by 12...
It makes no difference how many kids you've got or whether you're married. You can expect to take home approximately 70% of your gross pay.
By the way, you don't work a week per month for nothing, you get 1/12th of your annual pay each month.