We had that same issue when our boys were little. To make things worse, a child's safety seat can take more space than an adult.
I bought doner bench from a friend and hacked off the arm rest part of the frame. That gives a bit more space. The wheel wells make better arm rests for a kid anyway.
I did have to add some reinforcement to the frame to replace that of the armrest tubing.
After I got done with it I gave it away to another young family. They used it for a few years and gave then away too.