The market for VR headsets is worldwide, so the right denominator is the ~600B children in the developed world. If you generously figure that half of those headsets are used by children, that's ~2% penetration, which seems more like it.
20M units sold is tiny for a consumer electronics product, BTW. I work on Android Tablets, and we have ~300M MAU. Phones are 3B. 20M makes VR only about 20% of the market size of AndroidTV, which has about 110M units sold.
