My experience is exactly the same as mugsys_m8 – Chariot CX1 in my case.
Started using it when they were too small for a helmet anyway. When we did find a helmet small enough it pushed his head forward too much for comfort.
The main risk seemed to be striking his head on the frame of the trailer and when he was still small there was very little chance of that happening as the 5-point harness held him in place so well. In other trailers that don't have such a "comprehensive" harness (eg Burley) I was keener on him wearing a helmet. And also keener when riding in traffic, rather than just off-road.
We rolled that trailer several times with no ill-effects (and no helmet). Once or twice he didn't even wake up!
As he got bigger it made more sense to wear one as the chance of bashing head on frame was higher, and he wanted to wear one as he saw us wearing them. Also we often had his bike strapped on the back too so were taking a helmet anyway.
Just starting on number two (4 months old) in that trailer – first ride was on Sunday.
Look at the trailer and the child and assess the risk depending on where you're riding.