Make sure that they have something to help them pop their ears during take off and landing -- such as a box of juice to drink. Your older child might be able to chew gum. This is the key to a happy flight with kids.
Bring healthy snacks. They don't serve much food on airplanes these days, and if your first flight is dealyed you might not be able to get anything to eat during your layover. Make sure you bring everything you need -- spoons, napkins etc.
If your younger child is in diapers, change him at the gate right before you board and you might be able to avoid changing him on the plane. BTW, I don't think that preboarding is all it is cracked up to be. Who wants to spend an extra 20 minutes on the plane with little kids????
Read them a book about flying. We have a really simply one by Byron Barton called Airport or Airplane or something like that. Don't count of being able to read to both of your kids at the same time on the plane, though, it is much noiser on airplanes than I ever noticed before I tried reading alound to two kids!
Since your DH is traveling with you, you can trade off the kids. You help keep the more difficult child amuzed for a while, and then switch so that you can have a break with the easier child. This seems to keep everybody fresh and break up the time.
Sticker books are great, mellow activity. My kids like the Sticker story books. A box of crayons and a new coloring book is always nice too. I don't let my kids have coloring books at home because I think it is better for them to draw on blank paper. The cool part is now my kids think coloring books are a hugh treat, saved for things like flying and family emergencies.
If your 4 year old can use a walkman, he could listen to books on tape on the flight.
I would try, really hard, to relax and have faith in your kids. Airplanes and flying are very cool and interesting. Keeping talking to them about where you are going and why, and show them the maps in the airplane mags. Your kids can understand a great deal about what is going on and most likely while enjoy talking about it.
Also, the airport is a great place to burn off a little energy before and between the flights. It is a lot of fun to hang out at the gate watch the planes land, the bagage get loaded etc.
Enjoy your trip!