A 1-2.5 year old who is used to sleeping in an FAA-approved car seat will typically sleep better/more easily in the car seat than with CARES. That said, sleeping flat may keep some toddlers sleeping longer/better in-flight than any car seat. I would probably more strongly suggest considering using the car seat on a US-Europe overnight flight but maybe not as strongly for a Europe-US flight. But I also would suggest that sometimes it’s better and safer to be less encumbered carrying stuff around the airport and on and off the plane when with a toddler and then my relatives go with just CARES for the flights.