Do you know what causes the stress, gdl? Remember: you'll be together, just like at home. If something happens at home, you and your wife are probably very resourceful (like most parents) to find a solution for whatever problem. You'll bring that 'resourcefulness' to wherever you go, also on a plane: you're there and you just cope with whatever happens. I'm sure you'll be fine.
I have no experience with a long flight like that, but I remember the stress I had with cycling with my daughter for the first time (she in front of me on my bike in a 'bike-chair'; I don't know what they're called): what if we fall, what if a car hits us? Things like that. The only way to overcome that was just to get on the bike and go. The first few times were a bit stressful, but now I (we) really enjoy it. Completely different from a long flight, of course, but maybe part of the cause of the stress was the same: concern for the child(ren), fear that something happens to them.