Well, you are their coach so it's up to you, but I think if they are comfortable with their style and having a good time playing you should let them carry on as they are rather than remoulding their games. Many professionals past and present don't have a cast-iron perfect technique for some of their best shots, yet they are effective with it through years and years of practice.
Don't sweat it if they're struggling with their one-handers. Not many kids around still use it so I think we should embrace those that try.
They have fun in practice (beause they mess around most of the time) but they complain in matches asking me why they lose. And when I watch their matches, they completely dump their backhands, so what am I supposed to say? I use a one-handed backhand so I know how tough it is, but they aren't using backhands; they are trying to play racketball on a tennis court.
Professionals still virtually all follow the same techniques for every stroke. Obviously there are a few outliers such as Santoro, but even if you watch Santoro, the basic mechanics he uses are practically the same!