Need advice on talking to nanny about dropping 4.5yo nap

Let me start by saying I, along with DB and our 2 kids, absolutely adore our nanny and thank our lucky stars daily to have her. I trust her like family and give her the freedom to make all of the decisions while our kids are in her care. I have never pushed back on the schedule she has our kids on (although I’ve wanted to here and there, but they’re so minor it’s not worth the fuss), until recently as it relates to my 4.5yo son’s afternoon nap. He goes to preschool M-F until 1:30pm, and goes down for a 2-hour nap as soon as they get home at 2pm. We have a nest camera in his room, and I’ve noticed for the last 2-ish months that he is no longer napping, and instead just lays in his bed looking incredibly bored. The rare occasions he does fall asleep, he’s up and wide awake until 10pm at night. Also important to add he doesn’t nap on the weekends while in my care. I have made a few comments to our nanny about this, but she immediately pushes back to the point where it kind of catches me off guard. It’s gotten to the point where it’s really starting to irritate me and it needs to change for our son’s sake, but I’m at a loss on how to approach this with her so we both come out on top.