The Adams Trail-A-Bike does have additional attachments that include a seat back and strap to help keep a kid on the bike.
What you really might want if you haven't found it yet is the Weehoo
https://rideweehoo.com/products-weehoo-bike-trailers
Which the kid can pedal but is more like a lounge chair with straps for safety. There are weight limits for child seats, it isn't always that the frame, seat or rack can't carry the kid but more that past a certain weight the tail can really wag the dog if a kid chooses to struggle. Even with a trail-a-bike a shaking kid can make a fast moving bike unsteady especially under a lighter rider. From here you want to look at the family style tandem that has the adult in the back and the kid seated in front of the them cargo style, or modify a cargo bike, but it does start to get expensive.