Just talk to her about it. Frame it in the context of being a very small issue, just a personal quirk that you and your husband have, and a request of her - rather than a criticism. Tell her what you've said here - that you're so happy with her work, really value her, etc...
She may think you're a little nutty, but if you're careful about how you word it she'll likely think it's a simple enough request to try to accommodate. It's probably one of those unconscious things that will be hard for her to break though - so try to realize that she may not be able to stop entirely.
The other thing you can do, of course, is talk to your kids. "Daddy and I don't really like that word - maybe you mean you don't like it?" They're old enough to understand that different people use different kinds of language. They might even correct the nanny!
