And I'd never, ever tell a child after training what it's done wrong! Even in class, I try to work with positive reinforcement ("That's very good already, but could you maybe try to...?") instead of criticizing them.
Personally I don't find this so helpful, although I'm sure it depends so much on the details and your tone when you say these things. I don't like to make pointing out errors into such a big deal, either with adults OR with kids - that's what it feels like to me if you avoid ever saying something's an error or talk very carefully around mistakes instead of just saying that's not where that foot goes, etc. It feels like saying it's such a terrible thing to make an error we have to pretend no one ever does - but they still know they do so why not just tell that bit wasn't right but keep acting like it's normal and they just have to keep practicing and trying and in time they'll get it better and better? Of course it's good to focus lots on all the things they're doing right (especially things like being persistent and not giving up when something's hard!), and when you point out errors you point out things that are within their grasp and celebrate any time they improve at all, but why should it be such a bad thing if you sometimes do some part wrong? We're just learning after all, and there's no shame in making mistakes as you learn. That's part of how you learn.