I have had inertia reel seatbelts in the past that were temperamental to the angle they were pulled out . Especially if you try to pull it out too fast. With adults,not a childs seat.
I cant visualise exactly how your child seat operates . But would it not work better if rather than attempting to cross the childs lap whilst simultaneously unreeling you unreeled sufficient length vertically before crossing to buckle it up? . Any excess should rewind itself once buckled up. If you've tried this and it doesnt work or its not possible , I cant see any other solution .
I did think of feeding the eldest grandchild Easter eggs until large enough not to need a booster seat. But I suppose the youngest will also grow until the booster seat is needed again.