If you buy a no-expiry pass for your "under 10" child now, she uses a few days from it and saves the rest for another trip, that pass will be good whenever she returns to WDW, regardless of how old she is on return. All you have to do when you go back to WDW, is visit Guest Relations at any park and have them do the complimentary upgrade to an adult pass. Disney will not penalize a child for growing up.
But, you cannot "stockpile" unused child passes for the child to use after they turn the magic age of ten. If the pass has not been used for at least one day of admission, the best that you can do is upgrade the ticket to an adult pass for the difference in price between what you paid for the pass and the current gate price.