We looked into using trailer for our newborn, but decided to wait until he was old enough to support his head + helmet.
The alternative way would have been to somehow rig a baby car seat to the trailer. Around here most if not all infant car seats are designed to face backwards, so that should've been the orientation in the trailer as well. He wouldn't have needed a helmet in the car seat, but I didn't feel comfortable enough to start modifying our trailer.
Nearly two years later, he's riding in the trailer and has several hundred XC skiing kilometres under his belt in ski pulk designed for the purpose. In the trailer we can provide him with toys to keep him occupied, but when (not if) he drops the toys, the harness system often prevents him from picking them up again. In ski pulk toys are pretty much out of the question, as with all the clothing, mittens and whatnot he cannot do much with his hands (same in trailer when it's cold). He enjoys fiddling with the ski pulk visor though and recently discovered the joys of dropping a mitten in the middle of the ski track, for the later collection by one frustrated parent.
Key is interacting, taking long breaks to let him smell the roses too, and trying to cover the most distance while he's asleep. We can plan on only 3-4 actual riding/skiing hours per day. Combined with slower average speed that means really low daily mileages. But we try to make sure he enjoys the trips as much as we do.
--J