If you have the miles or dollars to afford it, always get an extra seat in coach. Even with a lap child, having the extra seat is a blessing and keeps you from torturing some poor soul next to you.

We now have three kids 5 years old and under, and we do an annual trip to Hawaii. That cost is over $17k in first class, so we snag seven seats for the five of us in coach (horrible 3-4-3 configuration). My wife and I don't sleep as well in coach on the overnight flight home, but the kids crash nicely.