This is not necessary the case and often quite the opposite. I've posted on this many times. My son has very flat feet and superfeet killed him and exacerbated his pronation. Is now 11 yr old and a very good, borderline AAA caliber skater. No special insoles whatsoever. Skating is not running, so, having something to "push their feet up in the arches" doesn't work in the same way it does with running because the ground reaction forces are different.
Honestly, recommending "high arch" insoles for somebody with flat feet in skating is probably about the farthest thing from what you would want to do. They will make most people in this category miserable. High arch insoles in skating are for people with.... high arches...
So, my point is, special insoles may or may not be necessary, but they are certainly not necessary simply because the skater has flat feet. Feel free to search my earlier posts on the matter, I've commented quite extensively on it.