vdbfamily: "I am interested in what the atheists who are commenting here would do if asked a question about God from a child that they knew came from a Christian family."
-- It depends on context and the relationship with the child (and age, too, of course).
As a teacher, for instance, I used to be very careful about even letting pupils know my opinion about religion/metaphysics/philosophy in general; 'I'll let you know what I think when you've left school ...'. (Same goes for volunteer school activities, obviously.) I used to find children appreciated hearing the reason for such a stance too (given at an appropriate age-level, again.)
For small children I don't know well, the issue never arises. Children I know well? Not many of these, really, but I tell them the truth if they ask me. That seems something of a duty to children as well as adults, no?
People in authority over children should take care in such cases, largely because of that authority and its likely consequences for developing youngsters subject to it. (Actually, come to think of it, I could mention a few religious people I've known who need(ed) reminding of this. So it goes.) Otherwise, though, 'tell the truth' is a good rule for life in general. If other people think what you say is wrong, get them explain why. That's also something to say to any child, at any age.