Try a Coverdell account. You can use the earnings, tax-free, to pay for private school. They work the same as 529 plans, but the latter has to be used for college. Also, with a Coverdell, you are not limited by the plan's investment choices.
You can set up an account for free at TD Ameritrade, then take advantage of the offer for commission-free ETF trades, including several Vanguard ETFs. Vanguard no longer offers Coverdell plans, as they try to steer you towards 529 plans.