There is no way you should do this. Not only are you clearly having reservations to begin with, but you need to keep your dignity intact, too. The whole affair is just going to be rubbed in your face. You deserve better than that.
While your ex may be coming from a 'good place' she shouldn't have asked, out of respect.
Your kid has feelings about the divorce but he's going to have to adapt to this new reality that your ex chose for him. It is her fault he's going to be disappointed, not yours.
I lived through this one, as a kid. It sucks all around. I'm now older than my parents when they split, and have an assload of insight into the matter. I carefully watched the whole thing unfold over 25 years, before I was out of the ordeal, living on my own, and away from watching the two people I cared about most be nothing but complete shitbags to one another.
Oh how wonderful, maybe at one point they can sit around the campfire, eat smores, and sing kumbaya