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.