I agree with the pp above. This is your second child and I assume you haven’t cared for another newborn or infant, daily, in over 6 years; it’s not unusual for a parent to forget the ropes on how to care for a child that young, especially if they haven’t in so long or only did it once in their life time. However, it’s very unlikely that a child caregiver with many many years of experience, will forgot how to take care of any child of any age.
I highly doubt that those who’ve been with those families that long will let you down. Do you know how many children those families have and at how young the children were when the nanny started caring for them? And I’m sure you have or will speak to those parents and they can give you piece of mind. I personally would go with a nanny that has dedicated many years to one family, caring for multiples then a nanny who only has recent newborn/infant experience and has been from job to job.