Why not just up your share by a dollar, or - if you are uncomfortable paying different rates -give a one year bonus and tell the nanny that both families will do a raise at the second family's one year mark?

Whatever we say could very well be disappointing to the nanny, so why not have the discussion with her, OP?

Question about nanny share and annual pay raise. The family that has been with the nanny for a year wants to give a pay raise, but obviously this means the raise will only be on half the nanny's salary (other family has only been in the share a few months). In this situation, how much of a raise is reasonable?