Yes, the pipes are zip tied for the time being.
I do have an idea to utilize U bolts instead.
The stand-alone cross bars are held in with the tension of ratchet straps. I am contemplating the U bolt idea for those also. Another idea is to weave a cord "platform" of sorts with 550 cord. The tension of the "platform" should be enough to hold the cross bars. I could also use 550 cord loops with "truckers hitches" (loops along the cord to add mechanical advantage for tensioning) to set the cross bars so they aren't an issue no matter what the platform does.
The bad part about U bolts is they add weight. So do the ratchet straps. Though, I think it is a really good idea to have ratchet straps for a multitude of purposes, if I can reduce them and substitute 550 cord that will save weight.
So the long and short of this trailer idea is it gives me a platform (no pun intended) to experiment with.
The downside to harvesting the parts from the kid trailer is they are weaker. The tires are 35psi max and I don't suppose the wheels/hubs/axle stubs are going to be very strong. I have used the kid trailer on some light day trips and so far they have held up. But with any real "touring load" they might be questionable. The tire pressure can be remedied with better tires. Whether it is worth it to put better tires on this wheel set I am not sure.