This problem is not unique to Tesla but this forum is. So, we're discussing the problem from the point of view of Tesla owners here.
Maybe Tesla could just engineer a properly-made, safe, seat-belt-extender-for-booster-seats solution and sell it to us at a typical Tesla markup. We'd buy one.
They just happened to make all vehicles with the same problem, not sure about the Y. I have a feeling it is the same.