I've been trying to think of a single passage in the NT which addresses people who are troubled by this question. I haven't come up with any yet. Anyone else got any? If not, how did it become a live question for many people in our churches? As a rule of thumb, I think that if our teaching and preaching and theologising is causing people to ask things which the teaching and preaching and theologising of the prophets and apostles did not cause them to ask, we should take stock and ask whether we're getting it wrong.
This question has been sparked by a series of articles about children (sort of starting here) which has made me think about my own children. They haven't yet asked me is they are Christians, and I kind of hope they never will. It's a question which inevitably takes our eyes off Jesus and sends us into spirals of introspection. (Do I have faith? Is my faith genuine? Am I growing or backsliding?)
I guess if they ever ask I will counter with 'did Jesus die and rise for sinners like you?' and leave it at that.