Ding ding ding! I think you hit the nail on the head.
What does OG always say, "Ignorance of the law is not an excuse for breaking it?"
In this case, "Ignorance of the camp is not an excuse for complaining about the camp's management."
If there was fundraising ahead of time, then they had a significant amount of lead time to attend. Also, I suspect this was entirely voluntary and the name of the camp and the program were not concealed. Now, it is possible that any Bible readings might have come as a surprise to the teachers, or, it may not. It may be one of those things that they have to go through to participate in the program.
Since many of these camps are run by Christian organizations, they can't be expected to redecorate the camp when the school kids arrive, and change the curriculum for each group attending.
As the father of three sons, and having also attended field trips with my children, I know that you can ask questions and look into the program being attended. However, the specifics of what the leaders might say is not likely to come out in any such inquiry, but if I were afraid that my children might be exposed to objectionable material then I would not let them go anywhere ... in fact, I wouldn't let them go to public school in California where they are confronted with objectionable ideas every day.
In the end, we come back to the same thing ... voice an objection to the school administration and then move on.
- Carl