What should I tell my kids about religion?
When children are raised in a religious environment, what they are taught about religion is relatively obvious and organized but what about kids raised in a non-religious environment? If you aren't specifically teaching your kids to believe in any gods or to follow any religious systems, then it may be tempting to just ignore the topic entirely.
That, however, would probably be a mistake. You may not follow any religion and you may be happier if your children never follow any religion, but that doesn't change the fact that religion is an important aspect of culture, art, politics, and of the lives of many people your children will meet over the years. If your children are simply ignorant about religion, they will be missing out on a lot.
Another, and perhaps more serious, problem with ignoring religion lies in how they will react to religion once they are old enough to make their own decisions. If they are unfamiliar with religious belief systems, then they will be an easy target for evangelists for just about any faith. Your children will simply lack the intellectual tools necessary to fully understand and evaluate what they are hearing, thus making it more likely that they adopt a very bizarre and/or extreme religion.
So if it is a good idea to teach about religion, how should it be done? The best way of going about this is to simply be as fair and objective as possible. You should explain, using age-appropriate materials, just what it is that people believe. You should also strive to teach about as many religions as possible rather than stick just to the dominant religion in your culture. All of these beliefs should be explained side-by-side, even including the beliefs from ancient religions now usually treated as mythology. As long as you don't privilege any one religion over another, then your children shouldn't either.
When your children are old enough, it might also be a good idea to take them to the worship services of different religious groups so that they can see for themselves just what it is that people do. There is no substitute for first-hand experience, and some day they may wonder what it is like inside a church, synagogue, or mosque better that they find out with you so that you can both discuss it afterwards.
If you are afraid that by teaching about religion you will also be teaching them to have faith in some religion, you shouldn't be too concerned. Your children may find this or that religion to be very interesting, but the fact that you are presenting so many faiths as equals, with none deserving of any more credence than any other, makes it very unlikely that they will uncritically adopt any of those faiths in the same manner as a child who is raised specifically to follow a particular religious tradition.
The more they know about the faith claims of different religions and the more sympathetic they are to how strongly every group sincerely and honestly believes these mutually incompatible ideas, the less likely they are to start accepting any one set of those claims to the exclusion of the others. This education and these experiences are, then, very much an inoculation against fundamentalism and dogmatism.
An emphasis on critical thinking is also important, obviously. If you raise your children to be skeptical as a general rule, it shouldn't be necessary to go out of your way to have them treat religious claims skeptically they should end up doing that on their own anyway. Skepticism and critical thinking are attitudes which should be cultivated across a broad range of topics, not something to focus on religion and forget about otherwise.
An emphasis on respect is also important. If, by example or design, you teach your children to ridicule believers, you will only be raising them to be prejudiced and bigoted. They don't have to accept or agree with or even like the religious beliefs of others, but they shouldn't make a point of treating believers as if they aren't deserving of the same respect as atheists and the non-religious. This will not only save them from unnecessary conflict, it will also make them better people overall.