At first is was just that I needed to wait for P to be mature enough to handle the switch because she was so close in size to J, that I couldn't really make the switch without switching both, but she wasn't old enough to handle the responsibility of sitting still without the restraining presence of the five-point harness.
Then my excuse was that I wasn't sure how to adjust the seating so that all the boosters (M and E are still in backless boosters) had shoulder belts, but there was still room for the fifth child to sit and everyone had easy access to the buckles. I still haven't completely resolved this, but I think I have a plan that will work.
Today, P put to rest my last excuse.
As she and J buckled themselves into the car P said, "You know mom, even if J and I get boosters, we'll still be your babies."