You are right, Mick et al...very dangerous stuff. Called "confined space diving". On the Dive Team we dive in black out conditions, usually "open" water area. You almost never see anything, sometimes not even your hand right at your mask. When there are tight spaces, some of our guys have trouble...vividly remember seeing one come up during a search for a possible drowning victim that may have been about 20 feet underwater...you had to descend thru a sideways 80 foot oak tree that washed downriver in a flood. He didn't make that dive. I ended up going down about 10 feet into the tree that time to retrieve our boat anchor that was tangled in the tree...had to take my fins off to get thru the limbs. I pulled it off but it's not a happy memory.
That's the first I heard of a two mile distance..wondered how long it might be. That is super ripe for catastrophe. Blackout conditions and add tight spaces...lots of people could not handle it. Some people have trouble just breathing thru a snorkel, much less a mask. Panic comes very quickly in water and only takes a few seconds to be in a shat load of trouble.
Swimming those kids out sounds like a nightmare to me. In a clean, unobstructed setting with a full face mask and plenty of time to work up to it they could do it. Dark, dirty water, tight spaces, kids that might panic...bad situation.