Usually banks wont take any income from welfare, childrens allowance, maintenance etc into consideration. they will however take maintenance payments into consideration if it is outgoing....
Depending on the lender I think they usually assume a certain amount of your disposable income is used up towards each child each month and therefore cannot be used towards a mortgage repayment. I'm sure this differs from lender to lender though...
We were advised not to mention any children when applying for our mortgage.
Having a child/children does bring your affordability down.
AFAIK, banks do not consider child maintenance as an income source when considering you for a mortgage, spousal maintenance on the other hand can be considered.