Don't know of many coaches who'd be teaching youngsters that one WillyorWonty!
I've always found that it is generally quite hard to replicate taking one handed diving catches, but the best way I found is to have someone hitting or ripping balls into the katchet with you standing 10-15m back. This obviously works a lot better when the katchet is placed on a hard surface and the thrower/hitter is accurate. I would only do this drill at trainings though, and keep my game day routines as simple as possible.