Before the trip, your 4 and 1/2 year old might like to read or see the VHS on The Magic School Bus Taking Flight.
We flew to Hawaii over Christmas just recently. Our youngest child was 16 months old at the time. We have 5 kids ages 9, 8, 5, 3, and 1. Anyway, both our 8 year old and the baby had trouble with nausea on landing. Having a spare carry-on outfit for each person is a good idea because you don't know if some one might get altitude sickness or if something large might spill on your lap.
If you are sharing a seat with your 20 month old, happy meal time! Nothing could be more stressful. Disgruntled legs kicking everywhere and food flying. However, the stewardess can help you pace when you receive your meals so you and your spouse can take turns.
The toys you bring with you on the trip there will likely have lost some of their charm once they have been played with thoroughly on the trip, so consider planning ahead for some new surprises for the trip back too. Although there are Walmarts and Kmarts in Hawaii to resupply your carry on bags with for the flight home, they may not be easy to find.
Some airlines have individual TV screens with shows and video games galore. Some don't. I hope you are booked on a flight that does, they are fabulous!
Trying to keep the baby pleased will probably be a challenge. Hand puppets, face making, peek-a-boo, coloring, match box cars, snacks of all kinds, switching places with family members now and then, pacing the aisles, small musical instruments like a kazoo (no, I am not kidding-it is easier for passengers to listen to a mother playing a kazoo like a nut than to suffer through hours of a baby screaming), games like incey wincy spider, a sippy cup, oh...if you go to amazon.com and look up "toddler games" in their "books" section, you can find a great selection of ideas there. We have Wonderplay and it is a hoot.
As for offending other passengers, well, if you are sitting by a particularly cross person, that cannot be helped. We have traveled all over the world with our 5 kids and that has only happened once. And it wasn't because of my kids, it was because of me. I turned to toss my husband one of the kids' shoes and launched it right into a passenger's arm on accident. Despite my apologies they paged a stewardess and asked to be moved somewhere else. But then all the other families sitting around us kind of relaxed and got their kazoos and bells out and we just all started having fun and our kids all got together and played.
There is nothing worse than a plane full of children with parents who are afraid of offending the other passengers so the parents end up shushing their kids and fighting with them to be quiet. But there is something wonderful about a flight where there is love and laughter and everyone just rolls with things and has a great, memorable time regardless of all the chaos and mayhem.
Lastly, a good nights sleep for all of you before the flight is A MUST. And remember to request to board the plane first when you check in at the boarding gate. Airlines always let families board first if they know you want to.