This is an outlet cover that you can use to protect your kids from the dangers of electricity.
Cover consists of two parts. One of them fits inside the other. Combine them and place the cover inside your AC socket. Smaller part (inner) should stay inside and bigger part (outer) should stay on the outside. Then use one of your plugs to rotate (as shown in photo with my hand in it) the inner part thus lock/unlock the socket. That means you don't have to take the cover out to use your outlet, yay!
Important Note: When assembled, this part has 3mm thickness. That means 3mm in the way of your plugs. When you are using the cover, make sure plug fits tight in the socket! Consider avoiding plugging a high powered device to a socket with the cover. A problematic contact may cause serious issues, such as fire!
Note: only compatible with European Schuko style outlets.
Another note: This is not my original idea. There are commercial products very similar (probably better) to this.
Use the included FreeCAD file to tune the parts to your likening. Uploaded STL files are a little tight fitting. You may want to relax the the dimensions a bit.
I've used PLA but ABS is probably a better choice, because there is a rotating part.