I don’t think a child would believe, at least not until s/he ends up being exposed. S/he could go until preschool, around 4 maybe, hear about god from another kid, especially if it is a secular daycare or s/he might go until public school, if s/he lucky. Either way, in this society the child would be exposed. After that, s/he may end up scared from whatever was told to him/her by another child and adult. Maybe s/he’d talk to his/her parents and maybe s/he won’t, but exposure maybe cause him/her to think about god. After that, it’s anyone’s guess what happens.