It makes it very difficult when you yourself don't know what to believe. It would be simple for me to answer their questions if they were my kids, but they are yours. All I can say is think twice before you start selling them on the GOD, Santa Claus, toothfairy, etc. Instead of saying I don't know, why not say "well, there are two different schools of thought on wether there is a god or not. Let's look into it together."
PS. I hope you don't give too much weight to the emotional 'born agin' advice. These dear folks are long on emotional faith and short on common sense.