I've traveled with all three of my kids (5, 3, and infant)on long flights and my advice is to minimize the amount of gear that you have to carry. Your 5 year old will be fine in the airplane seatbelt if you explain how and why to stay properly buckled in. You could also try a CARES, the only FAA approved Child Aviation Restraint System. http://www.kidsflysafe.com/
Much easier than lugging the carseat onboard!
Here's the link to the FAA child safety guidelines:
http://www.faa.gov/passengers/fly_children/crs/
The airlines also require that carseats be fastened in the window seat of the row so that it does not block passage for inside row passengers. Also, check to make sure your booster is approved for airline travel. If it is not they will not let you bring it on board.
And by the way, checked carseats do not count as luggage and they will not charge you a fee to check them. If you do however, buy a case for it to protect it... it will get really beat up when they handle it and if it rips or gets damaged you will not get any money for it.
It is also a huge headache to board the plane with multiple carseats and most major airlines will not let families with small children board early anymore. So you are stuck boarding with everyone else, bumping into the seats as you go down the isle, feeling pressured by everyone behind you to hurry up and get into your seat, securing your children while you buckle in carseats all while trying to stow your loads of other carry on gear (toys, snacks, diapers, etc). Most times the airplane seatbelt won't fit through a carseat so you have to get a flight attendant to give you a seatbelt extender. Or if you fasten and tighten it, the buckle ends up getting stuck in the rear of the carseat where you can't reach it to unbuckle it. Oh I could go on with the headaches of airline travel with kids.
And I once traveled by myself pregnant with two kids in carseats. One in a britax marathon and the other an infant carrier. I sat in the middle seat so I could tend to both kids. My toddler needed a diaper change badly, but since I did not have room on a seat (flanked by 2 carseats) and the lavatory on the plane did not have a changing table (most don't), I had to crouch down and change him on the floor in my row. Not fun. Having the extra seat space for diaper changing is much nicer.
While my daughter was still 3, I did bring along her Britax boulevard (big & heavy) but I got smart and bought a Go Go Babyz travelmate that attaches to her carseat and it was easy to use and she loved riding in her seat through the airport. Well worth the money in my opinion.
http://www.gogobabyz.com/products-c26-gogo_Kidz_Travelmat...
The less you have to carry the better!!!
Oh and if your toddler tends to kick the seat in front of her, warn the passenger in that seat ahead of time and apologize up front. I've found most people are more sympathetic if you warn them ahead of time.
Can you tell I've traveled alot with my kids?!?!??
Have a great trip and good luck to you.