LumpSum Invest for 15 years

Hi We have some of 10 lakhs and want to invest 5 lakh each for our daughters one is 5 year old and other is 6 month old. We do not want to touch this amount atleast till they go to college at age of 18. my wife insists on doing FD while I am lookimg for something alternate like nifty index fund please advise what can be best