I’ve worked with many kids in preschool settings as a 1:1 and outsider- not hired by preschool. These were children with SN and instruction was pull out to work on 1:1 skills as well as addressing peer interaction/ play skills. Usually as part of EI, ISP, IEP, Private consultation, etc. for 2-4 hour increments at preschool setting.
I’m not a nanny though, so education/specialization may be the reason I’ve never had a problem doing this. IME 95% of preschools, public schools, private schools, play groups, etc didn’t have a problem with my being there. The small % that did were usually principal issues in public schools and parents fought their way around it.
Again, purpose of nanny being there likely makes a difference. I wouldn’t address it as I want my nanny to come to school with my kid. If you have legit medical reasons why your child needs additional support then I’d write those specific things out and start by asking the preschool head if they have the resources to address them.
Your nanny should also be prepared to meet preschool head, present resume if asked, show background check, fingerprinting, etc. and have enough social skills and knowledge of your child and their purpose for needing to be there to get through a 20 minute 1:1 interview. It’s a rare occasion to be placed in any place with children as an “outsider” and not have to jump through some hoops like this.
If you think your kid needs so much extra attention I’d look into IEP/EI to get nanny employed that way. Just asking Can my nanny join probably won’t cut it. If you have some legal basis to back it up you’re unlikely to be rejected.