Same as everyone else... wax them
and sharpen them
and keep them in good working order.
They don't need a WC tune, but good wax and clean edges free of nicks. This isn't only about skiing but also about them not getting cut when the pick them up. I'm sure you're like the rest of the ski parents that feel like their main purpose is to be the kid's personal Sherpa, but at some point, they'll have to pick them up. I went through this too
. It also brings in the understanding that gear needs to be tended to and needs to be respected. I remember seeing my daughter mash her race ski on the other ski's heel binding to take them off. I was shocked and said "Hey! I spent on hour on them last night!" She never did it again. She didn't know as like many kids, had no idea what parents go through to get them through the day (wash clothes, buy groceries, tune skis, etc.). She also (her choice) did learn how to tune, but didn't care for it but appreciated the effort that goes into it. Fortunately, I love to tune and secretly thought of her encroaching on my territory, so I was happy she didn't take to it
Even at 5 and 7, they can easily hit 20 mph even if they don't want to. You probably wouldn't let them ride a bike at that speed without checking the brakes and making sure the chain is oiled or had the right tire pressure. Clean edges will help them hold an edge and even if only doing a duck walk or side stepping on ice, it will hold there too.
It will also help the skis last so the gear the 7 y/o is on can go to the 5 y/o some day.
Have fun.
Ken