I have wrenched my back several times getting a child into a childseat from outside a car.
Today’s invention is a childseat attachment which allows the seat to hang securely on the window frame of a car door. This lets a parent place their child in the seat without stooping and stretching.
The child is then belted into the seat and the door closed, allowing the seat to be detached, through the open window, through the very small distance onto the car seat.
The car door can then be reopened and the childseat attached to the car seat via the main belt in the usual way.
Or buy a 4×4?
Trying to load a sleeping or resisting child into a seat inside any vehicle (without damaging heads, backs -or doortrim) is still a major challenge. I’d settle for a seatbelt attachment that 4+ year olds can reliably engage themselves.
Getting the child involved is a good idea. Once they’re beyond 2 and mobile, it’s much harder to lift them and encourage cooperation. Maybe steps or a fold-down ladder for the child to ascend might help the process?