Just a few thoughts;
First and foremost; Seat belts!
Please oh please do not install seat belts on anything that is open to the elements. Bicycles and motorcycles never have seat belts for a reason. If something truly bad happens, you want to not be attached to the trailer. I will give a couple of examples;
You clip a curb a little too hard and flip the trailer. With seat belts the kids will be attached to the trailer as it grinds to a halt, using the children's skin as a brake pad. If they are not attached, the kids will slow down much faster (less momentum to surface skid area) then the bike/trailer combo. Or they may just be (mostly) harmlessly ejected away from the trailer.
A car hits the trailer with enough velocity to actually cause damage (a well built trailer with a good rider can easily withstand low speed impacts without leaving the ground). With seat belts the kids are stuck in the trailer as it rolls/flips/skids or crumples. Without seat belts they should eject from the trailer and (with a properly designed and built trailer) go over the car. This is one of the major disadvantages to the manufactured kid trailers. If you go against a car, the trailer goes under.
Seat belts serve a few purposes;
They prevent you from continuing forward after an impact (not a huge problem under 20MPH)
They keep you in your seat in an accident (you do not want this on a bicycle trailer)
They keep you in your seat during normal operation. This is the only advantage you can have using a seat belt on a trailer, though if you read further I will help you solve this issue.
Design of the trailer;
I have been operating pedicabs for over six years and have used many different designs (including the recumbent shown earlier). All of the best design possibilities are used in a pedicab made by pedaltek (
http://pedaltek.com/Photo_Gallery.php ) If you take note of a few features;
High seats - allow for safer impacts with 'cagers'
20" 48 spoke wheels - nearly indestructible. They are also easy to mount (each wheel is independent, no axle). They also fit under the seat. Folding a rim on a corner is a bad thing.
Notice how the seat is tilted back? It forms a 90 degree corner, though it is not parallel to the ground. This keeps buts firmly planted in the seat using gravity and friction.
I know you said you did not want to use a seat post hitch, though I must argue that this is the strongest, easiest to control and safest was to attach something to a bicycle. Notice how the neck of the trailer comes down at an angle? This pushes weight down on the bicycle during deceleration. This allows for better braking and control.
Please note that I am in no way affiliated with pedaltek, or Gary (the owner). In fact Gary dislikes me. I just believe that he has put a bunch of thought into his trailers and thought I would share some of those for your own build.