Letting the child decide this is to allow an uneducated and uninformed decision with a 50% chance of failure. Their comfort with the instrument based on first impressions is the wrong way to make the decision. It isn't like writing. Last I noticed most of us use only one hand to write but two to play guitar (or whatever instrument).

If a stringed instrument student arrives for the first lesson already having played, I won't change their handedness. If they've never played before I strongly recommend right, for the reasons I mentioned earlier.

(Some things, the archery example has been used, don't depend on the hands but the eyes. The dominant eye is the better way to assertain which way to draw a bow, imho, but I'd be interested in other reasoning.)

The greater challenge is to play guitar with restricted use of the hands and fingers due to permanent injury or disease. I've had students with those concerns and they were able to make real and listenable music...but that's another thread I expect.

It seems to me that the whole argument depends on guitar "handedness" and I'm not convinced that ultimately one hand has it easier than the other, or that in any event the choice is made to serve the "better" hand.

Peace, Mooh.