Now if I were
using the a-f notation, she would be (reasonably) confused as to why *some*
numbers are unique, but *other* numbers are also letters.


You greatly underestimate the abilities of children. This sentence of yours is especially (reasonably) confusing, given that yourself attested some sentences above the outstanding mental capabilities of your daughter:

My four year old can grasp tonal
just as well as she could decimal had I been teaching that.

Actually, it's quite a common (mis)-conception, often even used as arguments for spelling reforms, that children wouldn't be able to cope with some idiosynchrasies of certain orthographic systems. Guess how they coped that far...

Szabolcs

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