If you put kids in them 8 hours a day, it would be a concern (although I don't think there is substantive evidence either way yet). I think there's a safe middle ground where using something like this for up to couple of hours in a day at most is very unlikely to have any significant effect. It's just not enough fraction of the time to cause a major impact, especially older kids (teens).
we are already several years into millions of kids using VR headsets. I would say it may not be "proven" but it's also well beyond completely "unproven" in that if there were major effects we would definitely be seeing them organically by now.
That's why Meta tells users not to let children below 10 years use their headsets. And those between 10 and 12 should not use it for more than 2 hours per day.
