Tamper-resistant receptacles (marked with TR, sometimes know as TRR) are designed to be safer than regular outlets by making it more difficult to insert foreign objects. That said, there are lots of examples of things that were supposed to be safer which turned out to be more dangerous (uk outlet covers, wing floaters, margarine, etc.).
I could imagine tamper-resistant being less safe if they encourage kids to use both hands to insert foreign objects (disabling GFCI) or parents to leave extension cords in permanently to accommodate damaged plugs (increasing fire risk). is there any evidence to suggest that they are, in fact, safer than standard receptacles in the real world?
Best Answer
TRR are required by the NEC as of 2008 and contain built in shutters that prevent the insertion of an object into a single hole at a time. I could say that its obvious that most kids are going to try to only insert a single sharp object into the outlet, but I don't have any study to back that up.
However, there is a widely cited research study done by the Biokinetics Research Laboratory of Temple University that shows how ineffective many of the plastic outlet covers are. This study was done way back in 1997, but these plasic covers haven't changed much over the years.
The main points here are that 100% of kids age 2-4 could pull of one of the types and almost half of 4 year olds could pull off all three designs in less than a minute.
Now, it's true that this doesn't directly answer your question with a study of TRR outlets, but it does show that "standard" child protection was not adequate. Since TRR's have a physical shutter inside and should then prevent a non-zero number of accidents, they are probably better than no cover or a cover that can be removed (or forgotten by an adult). This should help with your research if nothing else.
Additionally, this blog from Leviton states (without a citation... I know...) that:
How do they know? Well presumably because they design and sell the devices to hospitals, but some hard numbers would be good.