And yeah, when there are 2 car seats there is TECHNICALLY usually no rule that the RF car seat has to be by the window, but if they insist on it, which happens frequently, it's very hard to convince them otherwise.
Actually, there IS! Car seats have to go by the window and no one can sit in between two car seats. FAA rule, which is why the F/A's "insist" (you would too, if you risked a personal fine of up to $1000).
I guess it could go in the middle seat, if the window seat were empty but that would be total wasted space of the empty seat and with two children, impossible. Window aisles have 2 or 3 seats together (I've never seen an aircraft with more than that in a window seat).
The car seats can go in the middle section, as long as there is an aisle on each side. Again, no one can sit between them. They would have to go together, rf or ffing.
The luggage cart I meant were the compact, light kind. Mine weighs next to nothing. Something like this;
http://www.luggageandleather.com/Products/Remin-Kart-a-Bag---Flite-Lite-Luggage-Cart__C525.aspx
You can get cheaper ones too. Mine has lasted over 20 years and many car seats!
Also, for the baby carrier, I meant the good ones like Mei Tai's, Ergos or Becos. Babies can get clingy in airports and if worn correctly, they like them (unless your child doesn't like being held
at all). I used my sling well past age 3 and my babies were BIG and I have a history of back problems (much easier on the back than holding them with my arms).
It was absolutely a sanity-saver flying alone with three children between Europe and California twice a year! I don't know how I could have done it otherwise...
So you would have the little one in front in the carrier, if not walking (be sure to place him down whenever you're not moving for awhile) the backpack in back and you would be pulling the two car seats on one luggage cart while the older one walked. That could work!