I have more than one kid at the same Overnight Camp but they are in separate grade programs. Will they get to interact at all throughout the program?

Yes!   While they will have their own independent experiences and won't be in the same activity group (and everything that goes with that), there is plenty of time for interaction at meals, evening activities, morning and evening clubs, and any other all-camp activity or transition times.  There is plenty of time for siblings to interact and it's great to know that siblings are on camp together so we can coordinate and encourage them to support each other through their own adventures.