Potential Solution

After getting Aidan to co-operate with me, and taking more accurate and up to date measurements of his height and weight today, it looks as though we will be able to use the same seat for him that we can use for Caleigh, which is the highback Graco Turbo Booster. I really didn't think he was A) that tall(39"), or B) that heavy(40lbs), colour me surprised. Anyways, all that remains is to make sure that we can fit the two of them in on either side of the Britax(and get the car repaired). I'm keeping my fingers, toes, and anything else that I can crossed.