I don't know how you format your resume, but out here in California we have to put a lot of detail in them. I would just list each family separately with the dates you worked for them. Then in the description you can explain.
Something like this:
Smith Family 1/2013- present
New York, NY
*Full time nanny share with Jones family. Caring for two children ages 4 months and 16 months when share began.
Jones Family 2/2012-present
New York, NY
*Full time nanny share with Smith family beginning in 1/2013. Cared for two children ages 4 months and 16 months when share began. Full time nanny share with Marks family from 2/2012-1/2013. Cared for two children ages 17 months and 2 months when share began.
Marks Family 10/2010-1/2012
New York, NY
*Full time nanny share with Jones family from 2/2012-1/2013. Cared for two children ages 17 months and 2 months when share began. Full time nanny share with Cohen Family from 10/2010-2/2012. Cared for 2 children ages 2.5 months and 3 months when share began.
Cohen Family 10/2010-2/2012
New York, NY
*Full time nanny share with Marks family. Cared for 2 children ages 2.5 months and 3 months when share began.
Obviously my dates and children's ages are made up and I'm too tired to make sure they all match up, but you get the general idea.