My son is 31lbs and 37" tall.
Have you tried cleaning the buckle out and see if maybe its a build up of gunk in there? You can use a pipe cleaner to clean it out and rense it out using warm water. Maybe that could help. How much room does he have in his current seat? How much growing room does your dd have in her Marathon. Is it a girlie seat? How long do you want to keep her in a 5pt harness? Do you like her Nautilus? If so I would buy your son a Nautilus for your car. You could pass seats down when your dd is older if its not a girlie seat. If she outgrows the 5pt harness at 6 you could pass that seat down in the least used car and buy her a booster only seat. Its just a thought. Dawn
DD will stay in the Marathon in the minivan until it expires in November.
I love our Radian.