Hi,
Some important questions... How old are your other kids and how long is the flight? If your flight is short, then holding your 6 mo. old will definitely be cheaper and doable. However, if your flight is long, it may be more comfortable to buy your child a seat and install the carseat so that you (and hubby) and child can sleep on the flight (I'm assuming your child is okay in the carseat). Watch out for the ear pressure thing, my daughter used to always cry (especially on the descent - she was always at her crankiest if she was sleeping and we began the descent and the pressure bothered her and woke her up - then she couldn't follow directions like "swallow" or "chew on this ____"). For your older kids, you may want to purchase Ear Planes or something similar for the ear pressure thing, for your baby, I think if you have their formula that they could drink that would help with the ear thing (maybe purchase water after you pass security and have those instant formula mixes ready). Lots of snacks helps immensly with the kids. I looooooove the portable DVD player (make sure your battery is charged up). Our trip to WDW (when my kids were 5 and 7) was great despite flying for over 10 hours (the first leg of the flight was red eye so they slept - the second leg of the flight they watched a DVD). My other friend sometimes wraps little gifts and gives them to the kids during the flight to make it more fun and to keep them occupied (e.g. books to read, coloring book, crayons, deck of cards etc.).
I guess that's all of my suggestions. Hope you have a great time!