My boss wants to change my hours?

I have a disabled child and I work 6-2 in a nursing home as a service associate ,a nurse messed up and lost her job as a nurse and now is working with me. My boss said that we have to work two week shifts from 10-6 because since we are working the same hrs she can't hire another person for the afternoon shift.. I have been there 6 yrs and I have to take my daughter to her school program before I go to work and pick her up after, I'm a her only caregiver.. I have to talk to her thursday about not being able to do this she knows my situation so why can't the other person just work the 10 - 6 sift any suggestions on how to go about this without getting her angry? Or losing my job?