If you give it to her Khums does not apply until she is a baligh. If you decide to keep it, You don’t need to pay the Khums if the difference is a few days from the Khums date. You cannot save it for years.
Here is the ruling from Sayed Khamenei
Q 882: Does khums apply to the money saved gradually intending to buy a house or other life necessities?
A: If, according to the financial status of the person, buying their life necessities depends on saving annual earnings and they decide to spend these savings to purchase such things within several days, then such savings are not subject to khums.