You can earn up to about £59300 a year and still get child tax credit so even if it is counted it may not make any difference. The benefits are to help raise the children so in any case should surely relate to cm not sm.
Anyway, if she didn't get the benefits you might be liable for higher sm, so don't complain.
I can advise that child maintenance is not counted when working out entitlement to Child Tax Credit and I believe that is the same for Spousal Maintenance (but don't quote me on that one). I will be checking that out tomorrow as I need to know as I am in the middle of filling in my form E. Will post the answer when I get it.