CC wrote: Not too fussed on what or specs.
Then you're probably in for a world of disappointment.
The coupling is the critical part of these, and to a lesser degree, the clamp. They need to have minimal play, or your kid will always be riding down the street flopped over to one side or the other like a drunken sailor... and the transitions from being flopped to one side to flopping over to the other can be quite violent and dangerous.
I have a Giant Halfwheeler (and no, you can't have it). I believe their coupling is about as good as they get, and I've modified it further (with shims) to get it even more precise... but it's still sloppy. I've seen some cheapies where the coupling is just a bolt going through a clearance hole in a thin-walled steel tube, and the "flop" angle is ten degrees or so... even before the hole enlarges with wear.
It disappoints me that none of the manufacturers have solved this problem acceptably. It's not that hard... it just needs a Hookes joint, which is a standard industrial coupling. Or some kind of flexible torque tube. Been done before, anyway.
I recommend you get a bit fussed on "what"... particularly in the coupling department.
who has towed that Giant many hundreds of km...