I think we shouldn't assume that we and our thoughts of them are the primary motivator in our kids' lives. Despite how it feels at times they actually are likely not living their lives just to spite us.
There is no actual, hard core, unquestionable evidence of anything supernatural. On the other hand it is logically impossible to prove that something doesn't exist. That means we do not know if there is something supernatural or not, it is solely the question of believing. Most people who believe in something supernatural do it for one of two reasons; either because of personal experience (which can be anything from earth shattering to some vague, whimsical voice/thought/hope/feeling nagging that maybe despite of everything, there is something else) or because that is how they were raised to believe and they have never questioned that belief. It can not be attributed to some kind of moral failing from their part if our kids have both questioned and thought about the matter and have not have any kind of personal experience that would have made them believe.
They may talk down our beliefs to spite us, but I do doubt they base their own existential understanding just for a hope to annoy us, at least not if we have not browbeaten them to share our own beliefs against their will when they were still young. Going to other way because of spite is likely for kids who have been brought up in religiously abusive environments (either at home or at church), but if that has not been the case, it is unlikely kids would harbour so much spite against the religion in which they were brought up, that they would deny it in spite. It is much more likely, that they simply do notice that they do not believe that way.
It is not always about us!
EDIT: I personally was brought up as an atheist, but some time during my late teens/early adulthood I started to question the belief system I was raised to believe and started to have that nagging thought that maybe after all... I ended up joining the church. I did not do that to spite my parents (though they were upset), nor were they in any way reason or motivation for that nagging thought. It was simply about me, my existential views and my faith, not about my parents. I ended up a believing agnostic (as in, I do have some faith or at least hope for existence of a deity, but I know absolutely nothing) which my parents do consider moral failing and something I did to spite them, but that is simply not true.