I think you could start a conversation *in person* with the mom, and say, "How do you like your nanny? We see her a lot here. How is she with your kids at home?"
And then you can say, "She seems extremely strict with your son. To be honest, I personally find her quite harsh with him. Have you been able to observe them together?"
Then leave it pretty much at that. You've given her the hint. If she asks more, tell her you *own* impressions.
I'd be so very tempted to
And I think I'd also start almost aggressively (not in manner, but in frequency) complimenting the little boy to his face and directly to the nanny. Before she gets a criticism in, "Oh, there's Joey, he's such a good little boy! You're such a cheerful and well-behaved guy, Joey! Isn't he sweet, Nanny? He's always so self-controlled--never out of line or rowdy. What a good kid!" Be specific to your observations: "Look how you're standing next to your nanny, Joey. What a good listener." And maybe even specifically ask him to come with you away from her: "Joey, come with me to look at this pictures they drew--Nanny, can Joey come over here with me to look at them?"
Start creating the words in the air that contradict her treatment of him.
I think I'd make it an absolute campaign.