bbm I don't think it's that simple. The kids are surrounded by water otherwise they would have gone further, right? The chose the only high and dry point there was. And there's no "hole" directly above them otherwise they would have been pulled out already. So that means any back entrance they will find will again lead to more flooded passages the boys would have to traverse to get to that exit to be taken out.
It's a pity these caves have never been properly surveyed. I'm surprised the locals don't know more about them, there must be at least some of them who have been in there. I read monks go in there to meditate?