While I might initially have the normal human reaction of being disappointed at the suggestion, I would QUICKLY welcome the opportunity to see how someone else might shoot my kids. I'd like to have some shots from the perspective of another photographer, for sure. (In fact, I have a good friend that has a studio. He's the guy I go to when I want great family portraits.)
Even if I was a world-class artist, and for the record there's no danger of that, there is a huge value to me in seeing the art of another. Seeing my kids through the lens of another photographer would be great. I'm pretty familiar with my kids and can be handy with the gear, but when it comes down to it their shots are going to be different for a wide variety of reasons. To see the end result would be fascinating, IMHO.
Another consideration could be the Mrs. wishing for a no-pressure situation for the kids. I know when I shot my Mrs or kids I am not as professional as when I'm doing something on the clock. I can't help it. Being too familiar with your subject changes the situation.
Now, is there any value to feigning insult and "allowing" her to to make it up to you? What's the next L lens you've been pining for? 