Agree with all these posts.. Rear brakes for the fixed if I were going to try to pull a trailer. Agree with debac that training wheels are the devil.

Two wheel trailers are more stable than 1 wheel stuff, but they are wide and it can be a jarring ride for the kids. Trailers should come with balloon tires imo. That gator looks pretty neat for being able to tow during the street stuff then letting them unhook and ride alone when you get to a safe spot.

You could kid back chop an old US tandem as described here- https://www.sheldonbrown.com/opctandem.html