The concept here is fairly straightforward. Standing (or on Xbox One, sitting!) in front of your Kinect sensor, a number of triangles will appear on screen, pointing in different directions. At the same time, a smaller triangle will appear somewhere else on the screen, and shoot towards the bigger one. When the two triangles cross, you have to wave your arm in the indicated direction. It's something that's a bit tricky to get across in words, but one that feels pretty natural when you get going.
For the most part, Fantasia is pretty easy to pick up and play. It's kind of like Just Dance, only instead of waving a Wii Remote around, you just wave your arm instead. The only real issue for younger kids is that, unlike Just Dance, it's not always obvious which note/move comes next. Because the prompts don't scroll from left to right - they just appear in random places on the screen, and the timing markers fly at them from another random part of the screen - it can sometimes be hard to tell which note you're supposed to be playing next. With lots of notes on screen, and no obvious patterns, it can sometimes get a bit confusing - although on the plus side, it doesn't seem to be possible to fail. For those who find getting through the game's main story mode too tricky, there's also a Party Mode that can be enabled in the game's settings, which unlocks every track in the game for you to have a play with.
Beyond that, the only other thing parents may need to be aware of is that the game does require a Kinect sensor on both Xbox 360 and Xbox One, and as such, can be a bit pernickity from time to time. Sometimes not noticing you've waved your arm in the right way, or punched towards the screen as you have to do for certain notes, it can be a bit awkward from time to time. Perhaps the biggest potential issue you might have comes from navigating the game's menus, which require you to push forward with one arm, then bring your other arm forward to meet it, before pulling them apart, as though you're doing breast stroke (which relies a lot on Kinect's less than perfect depth sensing), and the mid-song sections where you get to apply your own remix, usually by waving your hand around on a cube, or playing with a virtual string to mess with a synthesiser. While making the tune isn't too tricky, if you're playing sitting down, it can sometimes be a bit tricky to exit out of these sections, as you have to put your arm down to continue - and if Kinect's struggling to tell where your arm ends, it sometimes doesn't get it quite right.