Pack and Play, or just a sleeping pad with a bag. The first night might be a little tough to fall asleep if they haven't been playing outside all day and are excited, but the second night they will likely sleep just after dinner and not wake up till morning.
I've only every had trouble with camping with babies, tots and kids, is if they get cold, so if your anywhere at altitude or where the overnight temperature dips, bundle them up.
A test night close to home, or in your back yard can be a wonderful thing too.