That's a hard one. Her needs have changed, it just depends if you would like to continue to care for her child or not and fill the other two days or look for someone fulltime. I see it like this you could either
1) Inform her that's not a problem and let her know your part time rate (make it higher of course and maybe she'll stick to 4 days or agree and you are still better off as you can fill the other 2 days and come out a head.

2) Stick to your original agreement and let her know that you don't offer care for less than 4 days per week as it is not worth it to you financially.