I've seen a shuttered schuko receptacle advertised in an early 1960ies magazine, but never actually saw one.
There are child-proof covers available for sale. They're stuck into the receptacle and usually have an adhesive strip (remember, Schuko receptacles are recessed, so you just put a plastic disc into the opening).
There are 3 systems I know: Slide down, turn 90 degrees (both as C-H described, just the slide ones were a little bit better). They have a rubber band as a spring which naturally ages by time. We've had many of them around when my brothers were young. The worst (and probably oldest) type were just plain covers which were inserted and required a special key to remove them before a plug could be inserted. I've seen many of these covers but not a single key. They always get lost, so I got lotsa training removing these covers with screwdrivers.
