I just think that too often people consider "cheap pet" to mean "disposable animal". I'm not a fan of carnival prize goldfish, but I've seen them end up in good hands before. At that point, the fish went from unethical treatment to ethical.
While I appreciate the value of teaching children responsibility by giving them care of an animal, it needs to be supervised.
An example of what I consider "the wrong way" is a friend of my ex gf, who buys his daughters new goldfish every week or so to replace the ones who found Cheerios and tomato soup and whatnot unpalatable -.-
To make matters worse, last year he even gave in to her demands for a clownfish (I refuse to call them "that movie title" fish). And I'm sorry to say, he felt it would be a good roommate for the remaining goldfish. In an uncirculated, unheated... freshwater bowl.
That is unethical. And the daughter learns nothing, except to devalue life.