The problem with explaining the Bible to young kids is that it is very hard to understand, especially for someone young, not to mention that it describes disturbing situations such as incest, rape, murder, war, etc. etc. Also, a certain level of knowledge about the historical context is necessary to understand what it is even saying (“kick against the goads” and phrases of that sort), so you should probably just have them read specific passages that directly relate to basic Christian beliefs if your goal is to give them a general understanding of the religion; after that it can be up to them if they want to explore it more. Even still there is the question of which translation to use, as some of the easier to understand translation are questionable at best when it comes to accuracy.