Replacement transmission?

I have an '02 Nissan Sentra with only 75K miles and in excellent mechanical shape. I am going to get a new car. My children are becoming of age to start driving, and I would love to keep the Sentra for them to use, but my wife is against them driving it due to its manual transmission. I would like to weigh the cost of trading it in for a similar car w/ automatic, or to see if it is possible to replace the manual transmission with an automatic, and at what cost. I would go this route if cost-effective, again, because I am original owner and know the car is in great shape. Any idea how much that would cost, and whether it would be a good option?