There have been several instances of kids getting hit in the chest (infielders) and dying. This is because of the bats being so light/engineered that the ball comes off quickly, with a lot of force. I don't know if in the t-ball level you really need to worry about this, but I can certainly understand why any parent would worry. Sometimes improvement is not always for the better. These are not the bats of our youth. Compare a bat from 2 yrs ago, to a 5 yr old bat to a 10 yr old bat.
At the Tball level, nobody is going to die or get injured from a ball hitting them in the chest. Most of these kids cant throw the ball more than 20 feet and aren't hitting the ball much farther than that.
What you detailed can happen, although it is a highly rare occurrence, at higher levels. If a HS player wants to wear it, I could support it (although to be honest I never heard of it until I read the post above)....but putting a kid at 5 - 7 or 9 years old in that type of protection is just overkill, IMO.