I seem to recall that Wendy (who is a him!) looked at trailers vs child seats from a safety point-of-view for his children, and came to the conclusion that a trailer was safer (I forget the exact details, but a trailer is likely to be given more space by car drivers, and in accidents occupants have had virtually minimal effect, even when the collision has involved a car, the trailer was essentially just pushed along by the car).
I suspect I'd also agree with that from a stability perspective as well, since all child trailers I've ever seen have been two wheelers, so it'll happily just sit there when not in motion, unlike a bicycle with a child seat.
Whilst a good cargo bike would be an option, it's never going to be a cheap one. A Bakfiets or even Yuba Mundo isn't a trivial investment. A sort of compromise could be the Xtracycle route, which converts a conventional bicycle into a cargo bike (and the Surly Big Dummy is a non-conversion version of that).
There are also electric bike options for Cargo Bikes, there's an eMundo, and there used to be the StokeMonkey kit for Xtracycles (although they seem to have hit cost problems). Either option, a cargo bike, or trailer, does leave you with a lot more weight to move around, and some additional assist isn't necessarily a bad thing!
(Oh, and
Kidstuff may be a more useful place to look for information on this sort of thing).
