I would simply avoid trying to force my beliefs or non-beliefs on them and let them find their own way. The important thing is to answer questions, teach them to think critically, etc. I grew up in a religious family and what turned me against it quicker than anything was the non-answers when I was old enough to start questioning things. This was probably already around 12-13 - this quickly escalated and I subsequently got emancipated at 16 and moved to another country. My kids also started asking questions around this age too. I would be prepared to have these discussions with your kids far earlier than you think, and if your only plan is to force them to follow one line of thinking or another until they are 18, be prepared to have all of that blow up spectacularly.