Apparently, I'll be in the minority here; but I think childhood is a time for kids to be able to try things they think they may be interested in. If they give it a try and find it isn't what they thought, I don't see the value in trying to teach them that they have to stick with something they don't like doing. :/
Of my own three kids and a number of activities for each, there were only a very few times when someone wanted to quit soon after trying something. Most of the time, each kid would stick with something (like a musical instrument) for a couple/few years and then decide s/he wanted to spend that time doing something else. They all stuck with at least one musical instrument for years. My sons stuck with baseball for years. My daughter stayed in dancing for years (and she's 27 today and still in a dance company "as an extra thing"). In most cases, with things they eventually quit they'd still use, and learn, the thing on their own. (That's quitting the class - not the activity.)
I think people (kids, anyone) need to sort out what's a commitment and what's a fun activity or an "extra activity". They grew up to be people with multiple interests, but also people who don't generally quit things lightly. They have long-term relationships, long (enough)-term employment, etc. I don't know.. I tend to think if a person is forced into doing too much stuff s/he doesn't like as a child s/he may reach a saturation point and have a harder time sticking with what s/he doesn't like later. Well.... my parents let me quit whatever I wanted when I was a kid, and next month I'll be having my five-year anniversary on than HubPages (and it isn't as if I've been having a big, giant, blast for this whole time lol ).
I just think it's more important to teach kids the difference between when, and what, is a serious commitment and important not to quit; and when something is a matter of personal interest, aimed at either just enjoying the activity or else pursuing what is a natural passion (in which case, kids don't want to quit anyway). :/ Life is short. What, exactly, is the value in learning that we have to stick with things that aren't all that important and that we don't like? (particularly if someone has given something a shot, and a second shot and has found he just plain doesn't like it). School and work are one thing. Dancing lessons or clarinet lessons at seven is another.