I think that is right to let the children to believe in what they want, for example, when I was young my fathers never make me go to the church (Except for special cases).
At this point of my life, I would have liked to show me something more, and I ask myself what would be the best way to let children select in what they want to believe without imposing an specific religion.
EDIT: of course I want to know on little kids less than 8~10 years.
It is likely that at some point, children will naturally become interested in questions like "where did we come from", "why are we here" or even "why do people think there's a magical man in the sky" (It depends on who they hang out with.)
Probably the best way to let them pick without imposing, is by telling them "lots of people believe lots of different things" and then helping them find information on the various religions and philosophies. In addition to getting some books for them, it might really help to venture out and talk to some of these people, at least the ones in the area. Some will be easier to find than others, of course.
Travel with them and try to make sure the people talking keep it objective. It might help to not aim for the leaders but instead the regulars. Something as simple as visiting a local church service and trying to ask some of the visitors a few questions on why they believe might give valuable insights into the reasons people come to religion (instead of just talking about why you should join).
In addition, it might be valuable (and educational and fun) to grab some books on ancient mythology and look into how people used to answer these questions and what old and now mostly dead religions exist. If you can, you could even look for modern practioners of ancient religions (modern pagans and wiccans for example) to get a completely different look into how people deal with religion/spirituality in this century. However, these people can be hard to find in many regions.
I think there are deep and fundamental questions that most of us ask at some point in our life, such as "Who am I?", "Where did I come from?", "What is my purpose?", etc. Most religions, and even science and philosophy, try to provide an answer to these questions
I think it is best to let your child search for these answers on their own, by seeing what each of the religions (and non religious thoughts) has to offer. Tell them to be sincere in their search as well as through. And as a person of faith myself, I would add a small prayer to the effect of "O God, if you are out there, please guide me to you". I think answering these questions about purpose and identity is more important than simply choosing any particular organized religion (or even simply choosing atheism or agnosticism).
For a structured approach, the Unitarian Church has a good childhood education program that covers the beliefs of many different religions.
You can find this online at their web site. That's probably a good place to start in terms of how to introduce children to the variety of human beliefs without emphasizing any particular one.
