Trailers are more stable and affect the handling of the bike much less that bike seats. Trailers with chain stay (rear triangle) hitches affect bike handling less than trailers with seat post hitches. Trailers also have the advantage of being able to carry larger children (1-4 years) and multiple children. (several trailers are rated to 45 kgs. or 100 lbs.) Kids can be set-in the trailer with toys, books, drink, food and other amusements. The fact that the child is surrounded by so much entertainment and can nap in comfort in their royal coach, the child can accommodate longer trips. Trailers can provide more weather protection against rain and sun . Sitting low, with a low center of gravity, falls are rare and short. The typical brightly colored fabric used on trailers is very visible to motorists. The down side is that the child sit low and has a restricted view. And, trailers have a larger foot-print, which may be a problem on narrow roads and in congested urban areas -- instead of sharing a lane, you are more likely to have to take-up a lane for safety. But, rather than inciting harassment, trailers more often seem to generate friendly curiosity. The best trailers have the added safety features of a five-point harness and roll-bar. Face forward designs allow easier communication between the cyclist and the child, than rear facing designs. The child is also easier to monitored with the use of a handlebar bar-end mounted rear view mirror.
Tip: If you have a bike trailer with a chain stay attachment and are carrying two kids, be sure to put the heavier kid on the side where the hitch extends out. If you hit a bump or curb just right the trailer can tip. The seatbelt/harnesses are good, as is the roll cage, but if the heavier kid landing on the lighter kid, it may be bad. If the smaller kid lands on the larger kid, well, they usually just giggle a lot.
We don't know of any independent testing that has been done on child trailers, so we can't say which is best or safest. Things to look for are:
Is the child prevented from interacting with the wheels in any way while in motion?
Is the trailer stable, with a low center of gravity?
Is the child protected from sand and grit thrown up by the bike tires?
Is the hitch to the bike well designed and sturdy?
Is the point where the hitching bar is joined to the trailer (a stress point) sufficiently strong?
Is the harness for the child well designed and secure?
Other nice features:
Can the bicyclist communicate with the child easily?
Is there sufficient storage space to carry the child's supplies and other items (i.e. a bag of groceries)?
In some areas of Europe and Asia, cargo tricycles are used for transporting children.
Rough Risk Analysis Child Seats vs Trailers
Child seats create a higher center of gravity than trailers so in terms of physics and stability they have a high risk. But we haven't heard of enough accidents involving either to say that any higher risk is statistically significant. The smaller the child, and more proficient and safety conscience the cyclist, the more the risk for a child seat is reduced to about the same level as for a trailer. Road and traffic conditions can also affect risk. Some road and traffic conditions favor trailers and other conditions favor child seats. Road and traffic considerations may be a more significant factor than stability issues so it can't be said that one method is better than another in a given situation.