Gertrude
I'm sure the above is a light hearted comment but it does seem to suggest
that the requirement is not that important and any passenger enjoying the
view would suffice.
The requirement is not for an "observer" it is for a "competent observer" - which I would take to mean someone who understands they are performing a safety function, knows how to Look Out and knows what needs to be reported, when and how, to the "eyes down" PIC.
Whilst some teenagers do seem to think that they're immortal, there are others who take a sensible grown-up attitude to safety: I'm only suggesting you take one of the latter type.