It's mandatory in Catholicism that Catholics attend mass every Sunday (with "late" Saturday counting as part of Sunday), so attending mass will be very important to some. Given that you know in advance that it is important to them, it would be inconsiderate of you not to make arrangements for people to attend mass if they wish. If you do not have parental permission to take the campers off-site, that's a failure on the part of camp organizers. If you are allowed to transport them to church, do do on a voluntary basis:

Let the teens sign up to attend mass if they wish. If sign-ups are done without parents present, the number who want to go will be smaller. Ask for volunteers from amongst the adult counselors to supervise them. If you do not have enough adults, you can make it mandatory for them to accompany the campers to and from chrch, but not mandatory that they go in. If you have too many volunteers, you can require some stay behind at camp to supervise the campers. If the job description didn't include time off for church, you do not have to give it. The safety of the campers cones girst.