I'm in the "unless there is an issue (pain, performance, alignment)." don't mess with it. Unless there wasn't even a POS insole pad that comes with the boot, I might get an off the shelf one, but at this age, the kids still need a bit of room to stay warm and as soon as you get them something "just right", they grow and now they're in pain. I did get custom insoles for my daughter when she was 10, but that was because she couldn't stand being in her boots without them. She has hyper flexible ligaments and her feet collapse when weighted. This forced her to ski in a wide A frame to be comfortable.
More important than the insole, is the fit. When selecting boots for young skiers, keep in mind that they sometimes don't know what good is, they compare it to their comfy sneakers, and sometimes they fib (because they don't want to wear the ugly red ones). A good way to find out if boots really fit or not, is to take the insole out and have them stand on it. See pics below:
This is a very good fit but is probably pushing the envelope and would be the last season. She had no pain and her toes didn't get cold. She did get new boots before the season was out though.
This is the other side of the spectrum and it showed up in his skiing. He was inconsistent and tended to ski under his athletic abilities (very good hockey player that couldn't do a hockey stop on skis!?). They ended going back to his previous boots and this resolved many of his issues.
I coach U10s and the first thing we do each season is go through the boots. I take notes and pictures. I don't share any information with the athletes but that night I send everything to the parents with my recommendations.
Both pictures are of 8 y/os.
Have fun,
Ken