what I saw was a couple things, and occasional tipping in the shoulders (fairly slight, but tipping can be veru effective in powder), a little bit of stepping the inside ski (but I think the cause is the next comment), and a little bit of backseat. If you watch the snow flying off the ski, there is an abrupt spray in the mid to last part of the turn and if you look carefully at the end of the video, it might be from not enough flex in the ankles, causing the butt to stick out a little and some overcompensation in the hips (leaning forward with the shoulders). This could be causing the step because it looks like the hips may be getting a little locked (as I call it) and unable to flow smoothly into the next turn.
Overall though they looked like some pretty good turns, and it looked like they were having some fun. One question, did they know they were being taped? I find that people will try too hard if they are aware of being taped. Try to get some video of turns when they are just out skiing, and unaware of the camera, and see if the same things happen.
Honestly, I would have em try focusing on pressuring the shins into the front of the boots, and also worth trying is the lifting the toes to the shin (obviously you cannot do this literally), but try lifting the toes and forefoot up.
Just my quick observation(s) and an easy idea to see if it helps.