How can I keep the nanny with our family for a couple of years?

Don’t take your Nanny for granted. Treat her as a family member not an employee. Be sure to have mutually agreed house rules to ensure that the kids or the Nanny are not caught in between decisions. Consider offering a raise or some kind of acknowledgement of their work after 3 months of employment. Also something after a year. Maybe a bonus at Christmas. Birthdays are important. Communication is the number one tool in keeping the Nanny. Treat her as you would like to be treated.