Trying to find a balance

Between mediocrity and excellence... with my kids. I have found that I swing from one side to the other. I expect greatness but then they don't give it their full effort so I drop the ball and figure they can't do it because they are too young. There has to be some nice place in the middle, where they are challenged and their efforts are genuine. I just hate it when they come to me with a question about their work that tells me they are trying to do the very minimum to complete the work. So do I try to get them to reach farther or do I let them settle? I think it might be one of those learned things... If they are never challenged then they won't like being challenged and they'll never reach beyond on their own. But I need to find a way to challenge them, allow them to succeed or fail, learn, try again (if necessary), feel good about doing something they didn't think they could, and then they'll want to be challenged again (or at least maybe it won't be so scary because they know they can do it with a little effort/work). Kids have to learn to work, just like they have to learn to walk. I need to know when they are putting forth true effort so that I can praise it, and when they are trying to slide by so I can give them an extra push to try a little harder.