A cover with a simple key (a rod of particular length) (tm, that's my patent) could make a cover a cheap and ALLOT SAFE solution. However: covers have to be open with plugs. Even the "tamper proof" are not proof against that (the chord is exposed to scissors. partial pull-out leaves exposed prongs THAT ARE ENERGIZED IN FACT)
(1) you still have to watch your kids and be responsible if you FAIL
(2) a PHOTO (the one above appropriate for holloween) place at every outlet should convince the child it won't be a pleasant day if they inquire within
That would be my ruling. Something like a "Mr. Yuck" sticker and perhaps a cover BUT ABSOLUTELY NOT a teknorogy that requires rough housing the outlet: absolutely NOT.