As a parent and a former teacher, I completely understand and support the ideas of "learn by doing" and "keep trying until you get it right." After all, that's how you learn.
But as your customer, I'm tired of being inconvenienced.
Your helper has been here twice to fix the dripping connection under my bathroom sink after he installed it a week ago. You might want him to learn to fix his own mistakes, but I want to be able to put the toilet paper and Comet back under the sink where they belong (instead of in the hallway outside the powder room).
I haven't yelled and screamed at the kid. (Yes, he's a kid. I think he might have started shaving just last week. He probably just graduated from Vo-Tech.) I've just shown him how, 20 minutes after you run the water in that sink, you find a little puddle in the bottom of the vanity.
He keeps trying to fix it, and thinking he's done that. Then he leaves, and a little while later I find water under there again.
It's time to stop sending the helper.