I vote Nanny for the first 3 1/2 years at least. My DD has been in Mother's Day Out since she was 15 months and loves it, but it is only 10 hours a week in the fall and spring. She is very tired when she gets home, even though she takes a great nap. The teacher/child ratio there is 1 to 3. Any more than that I would be concerned that my child wasn't getting the attention she needs. (just my opinion, not to start a debate) Your son may be laid back right now, but just wait another 6-12 months!! It is during this time that you need to have some serious control over how cartakers are disciplining Jaxson.
A note about Jaxson thinking of her as mommy.....
It is very probable that he may refer to her on occasion as "mama" or similar, ESPECIALLY since her daughter will eventually call her that. Don't be hurt. I was a nanny in college and both kids called me "mama" on occasion but as soon as their verbal skills developed at little, there was no more of that. Plus, even if they couldn't call me Liz, they knew I wasn't mama. Basically, if a child can only say a few words, it only stands to reason that more than one person can be called that. Tell her to go by something as easily said as Mama, such as Nana or Dee Dee.
Both my kids called my mom "Mama" but I knew with time they would get it right!........and sure enough, they did!
All the best, Liz