Don't you all think that the reason for leaving, is more pertinent than, when? For instance, if nanny left in a huff on a Friday, and never showed-up again, even after two years on that job, is more of a red flag, vs. after ten months, the family felt that Grandma should move-in (to be closer), and Grandma was able and eager to care for the child?
Sorry but you have to manage your job history and its not the responsibility of potential employers who have other candidates to give you the benefit of the doubt. 1 or 2 one year stints if you have several 3 or 4 year engagements would probably be fine but if your resume is a string of only 1 or 2 yr jobs, you just will not be as attractive as other candidates to an employer looking for someone for more than 1-2 years.
The nanny who left in huff also wouldn't make the cut because she would have a valid current reference.