I had a 30A 240V outlet installed to charge the car. We have a grandchild that visits 2-3 times a week. They make household plugs to block 120V outlets in the house but as far as I know there aren't any for large 240V outlets.
I bought a matching plug at Lowes, about $15. With the plug assembled, there is an opening, made for the wire, that needs to be blocked. There are slots in the wire opening designed for wire retention hardware. A coin, US nickel, will block the opening but it rattles in the slot. A disk of plastic was cut from a plastic milk container lid, slightly larger than the nickel. It shims the nickel and prevents movement.
This should childproof the outlet when it isn't being used.