I have a used Bell child trailer that I bought at a garage sale a few years ago. At $50 it was probably overpriced. However, I would guess I have around 1,000 miles on it and I've hauled my kids around everywhere locally with me.
I've even had my 40 pound 2 year old in it with 3 - 5 gallon buckets full of sand in the back behind the seat. 1 gallon of sand weighs 9.6 pounds X 15 gallons = 144 pounds + 40 lb child = 188 pounds in it! When I've carried that much, it's only for 2.5 miles, and I take it easy.
I'm not sure what the trailer is rated to carry. I've looked around a little bit on-line, but didn't find anything. I'm guessing it's rated at the typical 85 - 100 pounds. It's relatively cheaply built, like many of the Schwinn trailers (it looks similar to the Schwinn Mark 3).
In short, buy a used kids trailer. It'll be cheap, it'll fold down, it'll probably have a rain cover, and it'll carry a heck of a lot!
One day, on my wish list if/when I ever can afford it: a BAW trailer.....