How Do I Choose A Bicycle Child Trailer?

Some factors to consider are safety, value for money, weight, durability and useability.

Safety: There aren't really any independent safety or testing standards.  In theory, most any lightweight cycle trailer will just fly up into the air (not too high) if hit by a car, instead of crumpling on impact.  But probably, the more you spend the more rigid the body of the trailer will be, which means that the trailer will hold its structure together better when it bounces back down onto the ground.

The same principles apply to most of the other attributes, the more you spend the more light weight, smoother running, strong the construction and practical to use the trailer should be.

That said, there's no point in spending a lot of money if you are only going to use the trailer occasionally or not travel far.  Most people who buy the budget trailer models (costing £100 or less) are quite happy with them.

Therefore the quick answer to this question is to decide on your budget, and then spend time examining trailers first-hand (at a local shop or even viewing at a cycle rally) so that you can figure out how different types attach to the bike, what extra features they have, what the cargo space is like, how the safety belts work,  how big (or small) a child can fit inside, etc.  Ideally you would borrow a trailer and try towing it, make sure you're up to the task of pulling it along fully loaded.