In at least some wide-body planes, a car seat in a middle seat wouldn't block access to the aisle and is thus allowed.

By the way, this kind of thing https://www.amsafe.com/kidsflysafe-com/ can work for some toddlers who are U2s eligible to be lap-children. Some airlines/airline FAs may make an issue about it -- more than even with some car seats indicated as FAA approved for in-flight use -- but by pointing out the FAA indication, questioning FAs seem to get around to their senses about these too being used in a plane seat available for the child's use. Even the non-US airlines.