If kids say it, I ask them what God means to them...who is God? Usually at that age, they don't know. I explain that if they don't know God, they needn't use his name. If they throw in your face that "you said it" ....(I try not to, but as you said sometimes we may not realize) I explain that "God" means something to me and sometimes things are appropriate for grown ups and not kids (within reason, of course!) There's nothing wrong with kids learning that!