I would have the baby up front, both of you will be more comfortable. 8 month old should sleep quite a bit any ways and dont have to be double miserable in the back. Then, if you want to come up from the back, you can just tell the flight attendant you are giving your wife a little break.
Unless you are talking about purchasing a seat for your child as well. If that is the case, I would skip buying a seat for the child and both sit up front and pass the lap infant back and forth.