I began installing these outlet covers over a year ago when my twins started crawling around the house. They were fascinated by standard outlet button covers, and learned how to pry them out. These sliding outlet covers are much simpler to operate (simply place plug tines in slots and slide to the left to engage actual outlet slots), and require a level of coordination that the boys cannot defeat even at 1.5 years.
The covers also solved an existing problem I had with loose outlets. You know the sort where you plug in your vacuum and with the slightest tug on the cord it pops out of the outlet. These sliding covers act as an anchor and hold the plug in tightly.
My house was built in 1991 and has what appear to be completely average outlet plates, and every one has accepted one of these covers perfectly. They screw in neatly, have a foam gasket for insulation, and the plastic is sturdy enough not to bow at all when firmly tightened with the screw. There are several brands of similar slide-type covers, but I can’t vouch for their fitting capability.