Read the entire article because if you take what I say out of context; you are an idiot.
You provide some very valid points as to why not to introduce 'feral' animals into another location. And to an extent you're right. Look at the Cane toad problem in Australia, it is causing huge economic backlash.
However stocking lakes? come on. I agree fully with Mjmc. And Ewrum; I doubt you can convince a child to kill a turtle he's cared for for the past year.
I'm not saying feral animals is a good idea. However I do not believe the garbage you are talking about with genetic diversity and stocking fish. Firstly, Darwin's theory states that for evolution to take place there must be a survival of the fittest take place. Secondly,Where I live in Canada used to be the bottom of a Mediterranean ocean. Evolution and ecological change go hand in hand. Thirdly, in some cases feral animals are far more profitable to the human welfare in the location.
Survival of the fittest, defined by Charles Darwin is an act where only the strongest of a species are able to procreate. If you take a turtle from pond A and put in an aquarium for a time either it will die from the diseases you speak of, develop an immunity to them, or lastly, you have listed VHS (Viral haemorrhagic septicaemia) has for the most part applied to farming rainbow trout. [
http://www.frs-scotland.gov.uk/FRS.Web/Delivery/display_standalone.aspx?contentid=739]
Hypothetically if you introduce that turtle to pond B located roughly in the same geographical area the other turtles will be faced with the same problem. Get through it or die.
Ecological change is splashed everywhere again and again [global warming]. Regardless the speed at which this event takes place you cannot deny it takes place. For example; See how the 7 continents used to be one. Central Canada used to be the location of a Mediterranean sea. Or the North Pacific land bridge. The worlds climate, environment is changing at a very fast pace. Furthermore this climate change is overshadows the possible lose of species caused by a young child reintroducing a native animal into another near-by pond.
Finally economic benefits to stocking certain locations with fish outweigh the possible loss of genetic diversity. Fish accounts for such a huge amount of the world's food. Even with the advancements fishing levels are considerably less then in the past. [
http://dieoff.org/page57.htm] Paint a picture of your glorious American wild west without horses. After all they were introduced into America and when the settlers didn't need them they were set free. Yet they didn't think the impact the horses could have on grass with their larger hooves, what they would eat, possibly irradiating various plants, leading to the irradiation of natural wild life.
I do not support the extinction of a species, but I understand new species are being discovered every day. Only now humans are being able to keep large scale accurate records of our impact on the animal kingdom. Humans have only been on this planet for what 200 million years? Maybe to the earth we are the anomaly and will be gone, our stay the equivalent in the blink of an eye. Don't try and take the joy and research that hands on experience away from kids. Having a pet turtle for even a week provides more information then books will, especially, to a child.