It shouldn't be adjusted that tight though, that concern would also apply to a child wearing a maladjusted baseball cap as well.

So I would suspect it's a minor concern, I would worry more about mental/cognitive effects more.

People have been doing that intentionally since prehistoric times. https://en.wikipedia.org/wiki/Artificial_cranial_deformation

> heavy constricting belt on a still-developing cranium

How tight are you making the VR headset? Whatever tightness you're using might be a bit much if you consider it "heavy constricting".