If she is getting worked up over it, might it be a good idea not to specifically make it an issue, but to introduce the concept gently and more frequently into life in general? "Can you cut that into sixths for me please?". "Could you just pour a third of a glass of milk?" "Can you divide these cookies up evenly among you all?" etc. I think tables are a good start, and you're right - it does just click at some stage. She is only young.