Are children growing up too quickly

My granny surprised me toady by asking me if Isabella still believed in Santa. As far as I'm concerned I don't think Isabella will find out until she is at least 10. Maybe that is unrealistic and as soon as she is around her school friends this will all change because as soon as one finds out,they will all do. Also children will learn from their older siblings as well. I really want Isabella to stay innocent for as long as possible. A friend today however said that this was unfair for Isabella as she would just end up disillusioned by life. But what is wrong with remaining young and innocent for as long as possible especially in the current climate where children can obtain guns as easily as penny sweets.