I’ve had issues like this before with my first family (who’s child I still care for & is now 18 months). Although it wasn’t in my handbook that it was, I gave notice that I was changing the policy to include mandatory naptime which would be effective on X date. The family knew they had two options: to accept it or to find other arrangements. I always inform parents verbally & then send out an email detailing & reiterating what I had told them in person so that there are never any confusions about the conversations are, & so that there is always a trail of these conversations being had. For emails that include significant information including changes to anything or anything I would think COULD be important in the future, I always ask parents at the end of the email to respond to the email acknowledging that it’s been seen & read so no one could ever say they never received an email.