“Granted, I am not a diving expert, but...”
The kids could panic. They could not know how to clear their mask and drown. They could go through a tank quickly if they hyperventilate. They don’t even know how to swim. At one passage the divers had to take off their tanks, something that is normally practiced and could have complications.They probably will not have fins due to the cave environment making the swimming slow.
As said above, an underwater extraction would be very challenging, , and pumping out vast amounts of water may enable a crawl out. But these are not your typical liberal American kids, and thus they are in overall good spirits and even laughing, and not suffering withdrawal from Fortnight.