Clif bars are my choice, though I haven't tried many others. My 5 yr old really likes them, & my wife will even eat them without complaining (not that she complains much anyway, but...) I tend to like the ones with chocolate best, but the apple & something-or-other was pretty good, too.

Here's their web page.

Though Colin Fletcher speaks highly of Bear Valley Pemmican bars, I found the ones I tried to be a little dry, and nowhere near the flavor of the Clif bars.

REI carries most brands, but other places may be cheaper.

David