The best way to childproof a door handle will depend on the type of handle it is. Round handles can easily be made virtually impossible to open, if you put a second skin (like a sock) securely round the door knob. This will leave your child unable to get enough grip on the knob to twist it and open it.
Almost all folding French doors will actually have lever handles though. You can purchase covers that will sit over lever handles relatively cheaply but there isn’t a particularly effective free way to do it.