@TJ - I am not saying that If I had taken them around Sherwood Pines a couple of times before they were 1yo that they would definitely have suffered any ill effects. I just wasn't prepared to take the risk and was happy to wait until I felt that any risk was negligible. Even at a later stage my eldest wasn't happy about the ride in the trailer saying he didn't like the shaking.
That said, the Chariot trailer equipped with suspension that jms mentions sounds like a great piece of kit. My trailer is crap and has none.
Mine are both beyond trailers now though. My 4yo rides better than I do and can't wait to get his hands on a DH bike.