We used the CARES system for my daughter when she was just two years old, and it didn't fit her real well. She is fairly petite for her age, but the strap that goes around the seat was not flat against the seat and it laid right at the base of her neck (so the straps would be at her shoulders). She couldn't lean her head back against the seat and couldn't get comfortable. She HATED it. For nap, my husband had to sit in a nearby seat so she could lay down across both his seat and hers, and we used the regular seatbelt just around her waist. I think if we'd had the carseat, she would have slept much better. We did use CARES again this last year (she had just turned three), and it fit her much better. We took a little travel pillow (not the neck kind) and a blanket, and she just leaned on the armrest/my arm and fell asleep - easy. But when she was two, I think she would have done a lot better in her carseat.
If you do decide to use CARES, check with the airline on the style of their seat. If it has a curve to it, where the sides stick forward more than middle/back, then the CARES will probably be uncomfortable.