I ran into the same thing traveling with a child. The child is big enough to only need a booster seat to use the seatbelt in a car but I couldn't get the belt tight enough on them because of the leather on the sides of the belt. I now know that even if your child is tall enough for just a booster in a car, they might not be safe in an EMS seat. We had to switch with people on an E190, and it was fine because they were happy and I was safe, but now I know not to let the agent at the Mosaic counter move us up.