The "zone of proximal development" is just beyond a learner's comfort level in terms of abilities or comprehension. It's the place were we grow intellectual and imaginative muscles.
This can be scary for parents and teachers. It's nerve-wracking when my kids climb a tree. Just now, as I was doing some last minute prep for my freshman English class, I had a moment where I wondered if I'd taken them a little too far beyond the zone of proximal development by reading Herman Hesse's Siddhartha.
But it passed quickly. Because I realized the fear was all about me and had nothing to do with them. It's true that we have to be realistic with our expectations, and we sometimes make the mistake of expecting too much, too soon, from kids. But the alternative is worse. Let 'em climb.