Dogsey

Nothing new... people will never learn. To leave unsupervised kids with dogs makes no sense.

'they found a piece of the dog’s tooth in his face'

A dog's tooth (especially of a large crossbreed like this one) doesn't just break off for nothing. There's an indicator that the dog might have been in pain, plus the heat on top and lots of kids playing around could indeed have provoced this attack.
But we'll never know as clearly there wasn't an adult keeping a close eye on it, and the dog has now been put to sleep so it's impossible to get an anwser.
It's just another number in a statistic demonizing dogs of certain breeds and a child and a dog have paid the price of adult stupidity.

How can they find "a piece" of the tooth, anyway? Teeth don't just shatter or break off into bits. No way bones in the kids face could have broken/cracked the dogs tooth, right? Kind of confusing TBH...

Yes they can if they are rotten .

Yes, teeth can break and fall out but for that, normally, there would have been some sort of problem first, ie gingivitis, infection, decay etc. All of those diseases can be very painful to the dog and can make him react to an approaching hand for example. But as I said, we'll never know