I tried this out with my 7 and a half year old at Xmas time, doing the appropriately themed Yuletide Journey. At first the rules were a bit daunting for him but he got the hang of it. When it came time to roleplay with the NPCs, however, he got embarassed and didn't really want anything to do with it. We ended up not finishing the story.
Today, my son now only a month away from being 9, we rediscovered Hero Kids and we played Basemet O Rats. He really enjoyed the rules and the combat, so much so that he wanted to play another adventure. So we broke out ye olden Yuletide Journey again. This time around he had a blast, though he still didn't really roleplay with any of the NPCs. He played along with the fantasy portion of it much more than last time, though, imagining the way the scenes looked and pretending to listen for clues.
I can say that, with a little prodding on my part, we both really enjoyed this module and finished it to the end. It is well written, though I think it could use a little more organization from page to page. Let me explain; it was sometimes jarring to read what came up right before a combat page. I would sometimes feel I was flipping back and forth trying to figure out what was supposed to be going on. But if a person has experience GMing for their friends, they would probably be fine.
My only reason for not being 5/5 is, as I said above, just slightly odd organization. The content itself is quite wonderful and I enjoyed the Xmas/Norse crossover :)