Kathy,
I also have not done this, but my impression is that others have mounted a switch on the seat run through a normally-closed relay. That way when the child sits on the button, it actually stops the car. The wiring is a little intimidating at first, but with a little guidance it’s not bad. The harder part might be figuring out the right switch. If the child can tolerate it, the easiest option would be an Ablenet Big Red stuck right on the seat. It wouldn’t be comfortable to sit on for long periods, though, so I think some others have shaped little seat platforms with switches mounted in them.
Happy to talk more if you would like. We can chat here or you can email me at janesw@health.missouri.edu
Edit to add: Kendra Gagnon posted a sit-to-stand guide back in 2017. I knew it was on here somewhere!
Enjoy!