'Coming out' as an atheist here in rural NI has been quite an eye-opener for me - people react very strongly to it. NI is a lot more religious than the rest of the UK though. Of all the people I know, there is only one other person who is an "out" atheist.
This causes problems for my kids at school (because they are taught Christianity as truth, not a church school, the curriculum here demands 2.5 hours per week of christian religious instruction.) It worries DD, who is 7 and very into believing things - if she asks, I tell her what I believe, but make sure she understands she can believe whatever she likes. But she is worried that I am hell-bound!
Secular activities are a Good Thing, IMO. I would send the kids, if they wanted to go, and I would not send them to a religious camp. Religion permeates every aspect of life here, and it is insidious and damaging. Atheism is not a belief system, not a religion, and IMO is a useful starting point for thinking.
I do not have a problem specifically with Christianity, I have the same feelings of revulsion for all religion and most woo - but Christianity is the thing that invades our lives every day, so my focus is brought to that most often.