I have an 8 yo & a 5 yo as well. they're fine back there, they love it. Fwiw, 8yo is 4'5" & 5yo is 3'6"

 

I got the Cosco high-back 5 pt harness boosters from Target ($40!). They are narrow enough to fit and hold my 3 year olds in the backseat.

As far as how they fit, their legs are too short where they don't bend completely at the knee when in the car seat so their legs stick out and they will complain "No room for my legs Daddy!" is a common refrain I hear along with "Move seat up!" I have to get them to sit with their legs splayed (Manspreading).

Once we get going, it's a totally different story. Anytime I rev above 5k they're both screaming their heads off in joy. And then I hear, "Daddy Do It AGAIN!". It's seriously so fun and worth the hassle of getting them in back.

 

Thanks for posting the picture, are the bottoms removable like in 997?

 

I think so but beyond my technical skill. I have found a link with step by step instructions.

 

In the 997 the bottoms are simply held in place by velcro, the backs are removable as well but you need to unscrew some torx. I was wondering if removing the bottom and the back will make more room for the kids in the car seat.