Off-Road Bike Trailer Suspension

This more as a guide to the rear axle suspension than anything else.  The rest of the trailer is irrelevant, build whatever you would like, but this rear design has worked well and has taken miles and miles and miles of pounding with no ill effect.  In fact coming down the Hiawatha Trail at speed my son fell asleep in his seat.  It rode smooth enough he didn't wake until we crossed the line at the bottom.  It really REALLY surprised some people as to just how smooth it rode.  You should see it crossing a cattle guard, the axle is a hammering blur and my little kiddo is just chilling in the back.  Empty the springs make some noise.  Cross between a creak and a pop.  Adjusting the coils so they are more inline helps.  With the weight in it it rides quiet unless the axle is really flexing up or down.

Parts!
Springs!  I used part number 1692K45 but I think the 3 inch would have been better.
SOURCE

Ball Joints!  I used the 1/2x20 for the trailing arms and 3/8x24 for the lateral bar.  I like the ones with grease fittings but they do make a mess.  I have no idea how long a sealed or nylon set would last, probably a very long time.  Personally I like knowing I can pump fresh grease in and force all the nasty out.  Keep a rag handy to wipe them off as extra grease attracts dirt.
SOURCE

Threaded Rod!  They have stronger stuff but I don't see why you would need it.  You can also use their stainless but I just painted mine.  I bought two 3 foot sticks of 1/2 20 thread (you have to match the thread to the ball joints you find, whatever they are) I used one stick for the axle and another cut for the trailing arms.  Starting over I would use 3/8 for the trailing arms, 1/2 is overkill.  I would buy one 3 foot 1/2x20, one 3 foot 3/8x24 and one 2 foot 3/8x24.  That would give you lots of flexibility in mounting your axle.
SOURCE

WHEELS!  I got these from Northern Tool.  You could source your own if you needed to.  I used 16 inch but have thought that 12 would have been fine and kept the cart lower to the ground.  Starting over I would go with the 12's unless you are planning on some serious off road, but if you are remember it will tip over.  I wish I could find dust covers for them, I have found some neoprene rubber pipe caps with a 2" ID that I think will fit over the hubs.  That would at least seal the wheels some.  You would have to cut a hole in the inside one for the axle and the outside one will have to fit over your mounting system.  McMaster only sells them individually and they are pricy at $4.66.  I have not bought them and can not vouch for their effectiveness.  I think I would try a local hardware or home center first but I included the part number for McMaster anyway.
SOURCE 16's
SOURCE 12's
SOURCE Protective Caps Part No. 2394K12

This has been a great build.  My son loves it and we have done 100's of miles in the original.  I decided to rebuild the unit to what you see here because my original suspension design allowed the axle to hop more than I like.  It was unnecessarily heavy duty and any weight I could shave off is good.  This new rear set up is both more efficient and lighter weight.  You can see the build log here: Bike Trailer and I have all the sources I used for the few purchased parts there and above.