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.
