When designing apps for children, the UX guidelines for Windows Store apps are generally applicable. However, there are a couple of additional tips that have come out of research on this demographic.
First, as with just about all new things, children tend to pick up the Windows 8 UX patterns more quickly than those adults who has preconceived notions about how the system should work. My six-year-old son is a good example here–he hasn’t once thought “this should work some other way” even though he’s worked with desktop apps as well. To him, it just is what it is, and because he’s primarily interested in the app, he doesn’t let other considerations worry him! In other words, don’t spend much time worrying whether children will be able to learn the Win8 paradigms that are represented in the design language: they just will.
Along these same lines, research has shown that children pick up on transient UI like the app bar after being shown once or twice. I think to them it’s something like a peek-a-boo game…which makes it fun!
As a side note, I’ll add that having a picture password is invaluable with pre-readers. I have an older Dell Duo that we use in the car for my son’s video player, as it has a 500GB had drive onto which I’ve ripped most of his DVDs. Before picture passwords, one of the adults in the car had to log in whenever we turned the machine on or woke it from sleep. A picture password, on the other hand, is very easy (and entertaining) for a youngster.
Finally, the other bit from research is that early learner apps should have primary commands on the app canvas and secondary commands in the app bar. For apps in general, the recommendation is that only “commands that are essential to a workflow” should be on the canvas, but for early learners it makes more sense to have more of those visible all the time. As an example, a video play command should be on canvas when the video is not playing. During play, other controls like forward and back can be in the app bar; again, as children will quickly learn to look there for such commands, they’ll find what they need.