We use a 'Little Van' trailer that we bought new from Ebay for about £60 - a bargain. I've made a few modifications. Firstly the towing arm now mounts directly to the rear axle of the bike. I use a bolt through on my SS and a DT RWS on my other towing bike, but it works fine with a standard QR too.
I have also removed the standard nylon fabric seat, and installed a custom made mount for my little one's Maxicosi Cabriofix carseat. This hold the seat firmly in place, even if the trailer rolls. The new mount is mounted to the trailer via vibration isolating mounts.
Our wee fella has been out in the trailer since he was 6 weeks old. Given that he was 6 1/2 weeks early, he made an early start. He has done 100s of miles including the GT blue and a gentle ride down Spooky Wood.
Last weekend we rode along the trail on the North side of the Dee, North of Braemar. This was a tricky tow, but a combination of soft tyres and a the vibro-isolating mounts ensured he slept soundly, even on the rutted stuff.
The gate to Rothiemurchies Lodge at the West end of Loch Morlich is too narrow, requiring the trailer to be emptied and semi-folded to fit through on it's side. Awkward, but not impossible.
At almost 10 months we're moving on to a Bobike Mini+, but the trailer will still be used for longer rides, if the weather is bad, or if he's due a nap.