I would check with the airlines concerned and receive a written reply before purchasing.
Many airlines however, do not allow them for aircraft safety reasons and I certainly would not want to take this through Miami airport either (as you could well end up carrying the thing miles on arrival to immigration).
As above, also your child must be small enough that they would fit the space, small children are generally happy to curl up and fall asleep anyway. Unless they are not good sleepers, I. Which case this would be no use anyway
We bought cheap £30 inflatables. As long as they can be taken down and don't interfere with the seat in front, no issue.
They made such a difference especially for DD9. To tall to curl up, too short to get feet comfy. We flew from Vancouver with 2 sleeping kids. The previous flight back from family in Florida I had two crying kids.
The cheap ones wouldn't create a stable bed, but was enough let legs rest easily and good enough to do the job.
The 1stclass kids version is only 25 EUR fills the entire void between seats and blows up rigid like an airbed. My friend has plane pals which are bulkier (has a foot pump) more complicated and pricier. Those bed boxes and slings look far too bulky and probably only good for tinys.