The only ones available when my kids were little were the friction insert type.
It didn't take long for my little ones to figure out how to pop them off.
So setting up my granddaughter's room I wanted to provide a better level of protection.
These from Safety 1st physically attach to the outlet. I can't imagine a child having the strength to pry or pull this out.
Yes, they are not easy for an adult to remove but isn't that the point.
Just grasp the side squeeze bars while depressing the top button and wiggle it out.
These outlet plugs will give you a high level of safety and peace of mind.