Are tamper resistant receptacles really safer?
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:


  TR receptacles have been mandated in hospital pediatric wards for over 20 years and have proven to effectively prevent electrical injuries.


How do they know?  Well presumably because they design and sell the devices to hospitals, but some hard numbers would be good.