Sure you can buy such plugs, but printing them is much more cool!
Once inserted into power socket they are impossible to pull without the key or similar tool. Your baby should be quite safe.
This model fits standard continental Europe sockets.
SketchUp files are there for anybody to edit.
Printing with 40% infill makes nice, strong plugs.
Use project planner to print multiple copies.
I added a tiny support for key "teeth" - just cut it off after printing.