Julia Galef, President and Co-founder of the Center for Applied Rationality, has video blogged on this twice. The first was How to Raise a Rationalist Kid, and the second is Wisdom from Our Mother, which might be a bit more relevant to you because, in that video, her brother Jesse specifically discusses what his mother did in situations where he wasn't enthusiastic about learning something. I should say that it has more to do with when your kids think that they're bad at things than with when they reject something out of hand. To that I would say, and I think many others would say: Kids are smart and curious, rationalism makes sense, and if they don't reject everything else kids have learned throughout history out of hand, then they probably won't reject rationalism out of hand.