Some atheist parents in very religious countries might have concerns about preschools / schools.
Reasonable to be concerned if you do not wish your child to be educated in religion or to attend on religious services.
Often there is the possibility to "opt out" your child of all religious activities.
In Germany you can decide wether your child takes the subject "religion" or "ethics". This is very easy, parents simply decide this and as schools mostly are managed by the state or city, there is no problem.
In Ireland you can opt out you kid of the subject "religion" but the default would be to take it. Problem there is, that your kid has no right to be supervised during the time it does not attend. This is because most schools there are church regulated.
But it is not only school, subjects that you might worry about.
People are talking. Kids can be cruel to each other when one is singled out on some activities (like in this case religious activities).
So you might wanna prepare your kids for this situation as well as for being asked questions or getting into conversations.
Maybe a few ideas - a bit of brainstorming - for parents:
When they are toddlers (about 0 - 2 yrs) there is not yet much to prepare them for.
Even if they are in a religious environment, they don't yet understand or process the concepts of religion.
But once they reach preschool and school age (about 3+ years) you want them to be in a mindset that is healthy and strong already.
A few questions and answers that might come up during kindergarten and school:
Q: Why can't/don't we celebrate easter,christmas, thanksgiving, xyz?
A: Because we do not share the religion. They believe in some things that we do not believe in, but we have our own celebrations that they don't have.
Q: Can we visit the church/mosque/monestary?
A: Yes, we can!
YES you can!!! Very important. Never make it forbidden, visit, explain things there, but do this from an "outsider position". For example "Look, this is where the Jews pray. The women sit here and the men sit there and look they wear those little hats. Do you see this roll, it is there scriptiure where they read their stories from. Like the book about animals we have at home. They read the stories to each other like we do at home."
Q: Why do they believe xyz?
Now this is of course very tricky, you as an adult know how but, explaining all these social, political and personal things is not only long but also boring for a kid. Something that might be a good idea, to actually talk to people who believe. Friends, preachers... But make sure that you are available for any questions after that, so your kid doesn't get confused about things.
Q: Why do we not believe in xyz?
First explain what "believe" in this context means, then explain why. Reason, proof, etc
Of course these are just some ideas, you might find better ways.
Something that sounds logical but lots of parents do not think about this for some reason:
Only explain as far as your child understands and is able to process. To tell a 4-yrs old child about social and political or even psychological things about religion doesn't work. They don't understand it yet. Keep it simple and the older your child gets the more detailed you can be when you explain things. You can talk about it with a 14 year old person in much greater detail. Don't force anything but stay available.
If you have more ideas, hints, experience feel free to comment.