You may want to consider this GoGo Babyz Kidz Travelmate http://www.amazon.com/gp/product/B000JJK9EY/ref=oh_details_o05_s00_i00 you could hook it onto the car seat and roll it like a stroller its perfect for the 21 month old . I've traveled with it and it's a life saver.
Last month, I did some major travelling with the Mishpocha - About 50 hours of flying time, spanning 4 continents.
At your suggestion, the suggestion of Dan (TR to FL), and others (sprinkled around DDF,) I shelled out the $$$ to buy the GoGo Babyz Kidz Travelmate.
In hindsight, this was not just an enhancement, but a GAME-CHANGER … worth every single penny.
I am just shouting it out in case anyone else is contemplating getting one ... if you are flying with an infant, it will make the world of a difference.
[Mods: This whole thread probably belongs in "Up in the Air".]