If they are clearly there because her bottom has been hit with either a hand or something else, CALL CPS!! Whether or not MB says they use corporal punishment, welts = abuse. Call CPS.
Now, if they could be from something else, I'd probably hold off. That's something you'll have to judge though. If you're a full-time nanny for this child you probably know their activity level and know if the "welts" could be from something else. Honestly though, welts on the bottom of a diapered child would be alarming to me.
A quick google search found this-
http://dc.gov/DC/CFSA/Support+the+Safety+Net/Report+Child+Abuse+and+Neglect