I’d be happy to charge mom’s iPhone for her
— Future Electrical Engineer

Best

Mommys Helper Safe Plate Electrical Outlet Covers

$3.16

Easily install a permanent and Convenient solution

These outlet covers replace your existing outlet covers and automatically cover the outlet when it's not in use.  Simply slide the cover to expose the outlet when you want to plug something in, and then it automatically slides back to the covered position when not in use.    

Pros
Very safe and super convenient

Cons
More expensive than plastic outlet covers


Better

Munchkin Plug Covers

$6.37

Simple and effective (though can be frustrating)

These outlet covers are the most commonly used solution.  Munchkin designed the smooth edges specifically to be difficult for young children to be able to grip.  They are a time-tested and cost-effective solution that works well.  The main drawback is how hard they are to remove (by design) which makes is inconvenient anytime you want to use the outlet.  

Pros
Cost-effective solution

Cons
Annoying to remove when you want to use the oulet


Good

Guardian Angel Child Safety Outlet Plug Covers

$39.99

Technology Solution for your "SmartHome"

These outlet covers actually detect what's happening in front of them, and automatically turn off the power when they detect the a person (or other 'conductive' material) in front of them.  We don't know a lot about the specifics of how this technology works, but they have been heralded as a great innovation in child safety, so if you can afford the price tag they may be a good option. 

Pros
Highly effective

Cons
Expensive