That is a way deeper hole than your typical kid digs with a plastic pail and shovel. Where the heck were the parents?!
I can’t imagine 2 kids under 10 digger something that deep, I suspect some one else dug most of it, and then left, and they thought it was cool and continued it.
Which is why they advise that if you dig a hole at the beach, make sure you fill it back up before you leave.
On an episode of “what on earth” they found that a rotting tree trunk beneath the surface can cause this type of hole. I think that story was about a beach on one of the great lakes so tree issue more likely.