Chariot sells several accessories designed for small children in their trailers. They state one product is for infants 1+ months old, and another is rated for 6-18 pounds. Go to
www.chariotcarriers.com and look at the accessories (Baby Bivy, Infant Sling,Baby supporter, bunting bag).
As far as Chariot trailers are as a brand, I believe they are the best ones on the market. We went through a couple cheap trailers with our kids. Once with a cheap trailer I dumped it over at low speed, and the seatbelts broke! So we looked at everything on the market, from Trek, Burley, etc., until we found the Chariots. They are a bit pricey, but looking back It's been one of our better buys.
We have now owned a Chariot Cougar 2 for several years, and it has been fantastic. The U shaped padded 5 point harness are great, as our son would unbuckle the cheap ones. The one downside is the time it takes to convert from 1 kid to 2, but we don't do that often. I also really like the suspension system. My daughter used to complain about bumps with the cheap trailers, that mostly went away with the chariot (Now she ride beside us). The hitch is also brilliantly simple.
The cheap trailers attached with a spring clamped to the frame; it jumped all over as you pedaled and scratched up the frame. The Chariots use a rubber ball, and a socket that attaches under the skewer. The first install takes less than a minute, and only seconds once the joint is on the bike. It pulls wonderful (no jolting) and doesn't damage the bike.
I don't know about the neck issue. To me it would depend on the child, and how strong they are. They would still need support for their heads so they can rest their neck. I don't know if you could get them to wear a helmet. Personally I might ride with them, once their necks are strong enough (not newborn), with appropriate support, in a trailer, on a bike path only (no cars).
Hope this helps