It is unlikely that the same nanny that you chose so carefully for your newborn will still be the best choice for your toddler or young preteen.
I have often found that someone who specializes in the care of a newborn is less energetic when it comes to engaging a toddler. It becomes a different type of job.
Much specialized knowledge is necessary for the proper care for a new baby. In addition to being knowledgeable, it is important that the person be very experienced. Often, part of the job involves having the baby nurse stay awake a good part of the night. We expect that a baby nurse will be very mature, and usually she will have had children of her own.
In the care of toddlers a different skill set becomes necessary for the job. It has been my experience that younger nannies who have not had children of their own yet tend to be very energetic and engage well with young toddlers. They enjoy getting on the floor and playing games, going to the park etc.
For some moms it is important that the nanny speak English very well. They feel that at a formative age their child should be exposed to correct grammar in order to lay the foundation for the child`s skillful use of language in years to come. But for other mothers the ability of the nanny to speak a second language is more important to them than having the nanny speak perfect English.
For the care of teenagers many moms seek a more mature person who the children will view as more authoritarian.
As your children grow up, the nature of what constitutes the ideal caretaker may change several times.
Of course I am making broad generalizations, and an individual may find that none of this pertains to them. However, these are practical considerations, which I have found to be significant over the years.