I used to get really wound up defending people's rights to NOT say "under God" in the pledge, let alone my right NOT to say the pledge or sing the national anthem.

And then I got a life.

If you're atheist and don't want your kid thinking there really is a God, fine. Take it into your own hands to teach your kid that. Otherwise, get over it. You're living in a nation whose populace is probably more Christian than not, being governed by people who are probably more Christian than not. Christian morals/principals/sayings are gonna leak themselves into official documents/pledges/etc.

If you don't like it, you can move.