I have heard of the CARES device, and it looks intriguing.
I can guarantee you though that most FAs have probably not seen them, and you may encounter difficulty in the crew allowing you to use them.
The installation video they show uses the device on an AA MD-80. It works fine with those seats because the tray table does not recess into the seat. The website says that on airplanes where the tray table
does recess into the seat back that the device should be installed
over the tray table, thereby making the tray table of the seat behind the child unusable. You may encounter some issues there
Personally, I would use a full-out car seat for a small child who still fits in a car seat. My 4 1/2 year old still uses his Britax Marathon (though he is approaching it's height limit). When I take him up in dad's plane, I use his Britax. If I were to take him on an airliner, I would probably just use the regular airplane seatbelt and leave it at that. My 3 year old is still very tiny, and I'd use his Britax.
I've seen many sit n' strolls used on planes over the years, they seem to work just fine on any plane I've seen one used on. Handy looking things, and it eliminates hauling & gate-checking a stroller.