Do United States military divers train in caves like this?

I’ve never heard of any military unit doing a cave dive in training. The risks are enormous and the benefits probably do not justify it. The closest thing in the Navy dive manual is what’s called “enclosed space” diving, which includes diving in shipwrecks or inside the ballast tanks of submarines.

For safety reasons, the Navy doesn’t allow divers to use scuba gear for those operations. Instead, it requires divers entering enclosed spaces to use one of two methods, both of which involve a compressor pumping air through an umbilical line to a diver wearing a full-face mask or a helmet that protect the entire head. Divers typically wear a compressed-air cylinder on their back as a backup source of air in case the umbilical line fails.

It might be possible to connect enough umbilical line to go the entire distance, from the cave entrance to where the children were, but pushing air through miles of hose would require strong compressors.

Using scuba gear, as the Thai divers apparently did, is riskier but was most likely the only choice available given the circumstances. There could be openings that are too small for a dive helmet, or it could be impossible to rig enough umbilical line from the outside of the cave to where the children were.

How could children who had never dived before dive now?

The answer is that you make it as simple as possible for them. A full-face mask, as the children reportedly used, does that. Typical scuba divers will wear a mask (like big swim goggles) that covers their eyes and will breathe through a separate regulator that’s connected to their air tanks through a rubber hose.