I will outline my particular theological disposition after the body of this post.

Advice:

On of the absolute best things you can do, both for yourself and your child, is, when asked the question, "Why does person X think Y?" is to go and do the research. Yes, this involves legwork, yes, this involves familiarity with the Dhammapada, the Q'oran, the Bible (and, as Catholics and Orthodox make up more than 66% of Christianity (and their systems of beliefs are far more subtle and nuanced than what you'll find in the Bible alone), you may want to consider learning about the Christian writers that they hold in high regard), and the Bhagavad Gita, but this is a question which will prove no small part of your child's politics, behavior, and intellectual development.

The next best thing you can do, is to keep asking "why". If someone says something disparaging about a religion, look it up. If someone says something good about a religion, look it up. Do the legwork and don't let prejudice stand in the way (that's lazy and disingenuous). Your biggest enemies here, in my opinion, are partial truths and mis-information1.

One of the things I try (and often fail) to do in religious debate is provide a well-reasoned, dispassionate defense of those with different beliefs from myself (as someone who has a family with at least three different doctrinal systems, this is invaluable). I've read all of the above works, as well as Nietzsche, Marx, and Hobbes so that I can try to speak the subtleties of faith. I do this because I've been in too many religious debates which have gone too sour.

1. So, recently, I was referred to an article where someone quoted the Pope as saying, "Theologians in the Church said that this scandalous behavior was not evil at all" when his real quote was more like, "These scandals are caused by a grave and profound evil. Unfortunately, some who claimed to be <insert original quote>, when, in fact, it was totally depraved." Unfortunately, people took the article at face value and then started a series of long tirades against the Pope...

Personal background

(Included because I seems to be the most theistic voice answering your question)

I think it might be a good idea to note first that I am a theist (Roman Catholic), I was born and raised as an Evangelical/Fundamentalist and a young-earth creationist (I once said that the Big Bang was an atheistic philosophy which should not be taught to my physics teacher... in the middle of class), for the first two years of college I professed minimal Christianity and eventually agnosticism. I then converted to Roman Catholicism my Junior Year, and I remain a devout one to this day.

My mother and step-father can be best described as Evangelicals. My father and step-mother are "liberal Christian" — that is hard to explain, but it has generally been summarized that the Bible is more of a "moral guide" and the vast majority of it should be taken figuratively at best. My mother's parents were Protestant (Methodist? It wasn't really talked about), my father's father was a Unitarian, and I only learned that my father's mother had been raised Catholic when I was an adult.

And if you think that this is complicated, you should ask about politics.