As long as she's up for the increase workload (and increased earnings, obviously), she may be fine with the change.
If not, you may need to search for less expensive options.

Just ask to sit down and have a meeting. Be honest with her about your reasons and what the new schedule with be like. Also be prepared that she may choose to leave and find another position working for just one family.

I have a full time Nanny for our children but due to my financial situation and that my children will be in school at least 3 half days a week, I will need to approach our Nanny about transitioning to a Nanny Share. Any suggestions on how to approach this with her? I feel terrible about doing this to her but she has operated in many share situations in the past.