After some good feedback on go kart safety from the Oppoverse, I ordered some 4 point harnesses and helmets for the kids (I still have a motorcycle helmet from when I actually rode for a year or so).
So yesterday we went over and my brother in law helped us add some posts to connect the harness. For the shoulder straps we added a bar to the back of the seat frame, then welded bolts to it. For the lap belt there were posts on either side for the single lap belt that was there, so we added posts to the main center rail.
My boys were great helping out, but I let my brother in law do the actual welding since we didn’t have all afternoon (his kids are 3 and 1).
Didn’t have much time to test it out yesterday, but I’ll need to work on something to keep the slack belt material from flapping around when the kids ride, but still leave enough so I can get the harness on as well.
UPDATE - A few better pictures of the posts and bumpers (also fabricated by my brother in law in about 10 minutes).