I never allow my children to go swimming with large groups unless I am with them. This includes summer camps, school programs or birthday parties. Two years ago the summer program at the school took the kids to an indoor pool. They had plenty of staff and there were lifeguards on duty. My daughter's best friend, who was 6 yrs old, was not in the water at all. She was apparently bumped by some other kids and fell into the pool. She was in 3 feet of water for less than two minutes and is now permanently brain damaged. This all happened within three feet of the lifeguard stand but there were so many kids in the pool that day he didn't notice.
http://www.bismarcktribune.com/news/local/article_e63e0e2...
No matter how closely the kids are supervised accidents still happen. Would it be possible for you to take some time off on the days they go to the pool? That way you can go with them and they don't feel like they are being left out. Or is the camp willing to offer an alternative activity that day for the kids who do not go swimming?
If you do choose to send your kids to the pool make sure they have life jackets on at all times and teach them what to do if they do fall into the water (Don't panic!!). You don't want to instill fear in your kids but they do need to understand that pools can be dangerous places.
And by all means, get them into swimming lessons. I found that it was easier to hire a lifeguard to do private lessons for my kids. Call your local pool to see if that's an option.
Good luck!