What an incredible discussion. It would be as simple as letting the kids hit a few doublehanders and a few onehanders to see which fits best.
Saying the doublehander is best for everyone is false. I know a lot of players who have a very natural onehander. Just look at the pros. Would Guga, Gaudio, Becker, Edberg have been as good with doublehanders? I don't think so.
OTOH, a good doublehander is a joy to watch. Rios, Nalbandian, Djokovic and Agassi are models for that.
I do think coaches are lazy sometimes in wanting to teach exactly the same game style to everyone, i.e. looping topspin balls. I for one was taught that game when I was young, only to find in my mid twenties that mixing it up with flat shots can be way more effective.
"Most of us believe in trying to make other people happy only if they can be happy in ways which we approve." Robert S. Lynd