Kid rides in passenger seat... significant other rides in back! (I do have a lap belt on each side couch, just like in RVs... they have worked well for 2 adult passengers)The bed is the easy part. A travel-safe occasionally used seat is the real problem for a grandkid. I certainly wouldn't trust anything I built or installed and wouldn't want even floor fasteners in the way the other 11.5 months of the year.
Maybe his mom would allow is to stuff him in a drawer.