Keep in mind that TR outlets are a relatively new requirement. A quick search shows 2008 in the NEC, which typically has some lag time for state adoption. Once adopted, the NEC has immediate effect on new homes and major renovations. But there are plenty of older houses - plenty of circuits are still without GFCI or AFCI or even grounded at all, because if it ain't broke, don't fix it. That being said, plenty of parents of small children will either retrofit TR outlets or, more commonly (since it is easier & cheaper) add outlet plugs when they have little kids crawling around. I did that ~ 20 years ago because it was clearly a really good idea. But those outlet plugs are long gone - as soon as I could consider myself safe because they were a real pain to use. On the other hand, I have one good TR installed as part of a kitchen renovation at that time (electrician put in GFCI, added more circuits, etc. to meet current requirements at the time, but TR wasn't one of them so the above counter receptacles are not TR) and it works great - though I have no idea if any of my kids ever tried to abuse it.
Now to some numbers. Well, not so easy. First report I found (from CPSC) seems to be (a) primarily about product (the P in CPSC) issues than about the built-in receptacles and (b) has low enough numbers that it is hard to draw any conclusions.
Aha! The Electrical Safety Foundation International (ESFI) has some interesting statistics for 1991 - 2001.
- 24,000+ children under 10 years old were treated in emergency rooms for incidents related to electrical receptacles - about 7 children per day
- 89% are under 6 years old
- 50% are 2-3 years old, the highest-risk group
and the typical injuries are related to sticking hairpins, keys and other metal objects into a receptacle. But wait, you say, doesn't it take TWO items to cause a problem? No, it doesn't. All it takes is ONE item placed in the hot slot of a receptacle with nicely moist hands and a finger (or any other body part) touching ground, which can include the center screw on many receptacles, as well as metal parts of many appliances, and now you have a complete and dangerous circuit.
Given those kinds of statistics, even a small decrease in the problem will have a significant impact on health & safety. It will take a while, but over time it should make quite a difference. Keep in mind also that the alternative of outlet plugs, if they even worked, and they don't work so well, as noted in another answer, was simply never used by many people. In my personal experience, typical thoughtful modern parents would use them. But most of the grandparents - who love to have the little grandchildren come over for a visit - never had outlet covers. Plus they are a pain to use for adults.
Could TR receptacles be better than they currently are? Yes. Most of the recent (a.k.a. mass market now that everyone must have them in new construction) are not nearly as good as the kind I have in my kitchen. But they are better than nothing at all and, I would argue, generally much better than outlet plugs.