BJ is right--it is rude to disturb others by repeatedly exchanging seats in a premium cabin, especially with a child.

What I would recommend if your wife's upgrade does not clear is for you to fly one way in Business Class with your wife and child in Economy and for your wife to enjoy Business with you and your child in Economy for the other way.

You mentioned that you want to get work done on the trip so if you are alone in Business Class for one flight, you should best be able to complete some of that work. I too like to use a transcon flight in a premium cabin to get work done. Thus, it seems reasonable that unless your child is extremely quiet for the entire flight, if you take her into Business Class she will be disturbing other passengers who like yourself are trying to get work done and who may have paid a premium price to be in the C-Cabin.

I say this now (and watch me do just the opposite ) that I shall never take children of my own into a premium cabin unless I am sure they will be well behaved and not disturb any of the passengers around them.

In any case, have a nice trip!