We have had the same nanny for almost 3 years for our DS who is about to turn 4. She is an amazing woman and is excellent with our son as well as all the kids he plays with. He is now in school for half a day and his friend's mom has asked if we would be willing to do a nanny share for a few hours a couple of days a week. Our kids get along very well and I get along very well with the mother, so I don't see any problem from that perspective and our nanny is happy to do it. Plus we don't mind saving a little money given our son is also in school for several hours a day.
Our nanny would pick up both our kids from school two (maybe 3) days a week and she'd have them together for 3-4 hours for those days (take them to the park, zoo, etc). We currently pay our nanny $20/hour for a 40 hour work week. What would be a good hourly rate for those few hours where she has both kids?