He does understand the pitfalls of opening the door and I have STRICT rules that the ONLY person allowed to open or close sliding door is me (a potential finger-chopper if ever there was one) and to be honest I'm reliant on him obeying all these rules but a belt and braces approach (if possible) seems no bad thing as I'd be left feeling wholly to blame if anything happened to him. You'll have to forgive me a little as I'm a much older Dad (him 5, me 50) so have to double-check everything I do now as my own response time is not what it was.
Lock covers sound good.
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How about the idea of a lock button cover as suggested, could be made out of acrylic and counter bored to sit over locking button, with a reduced base to go into door hole.
problem is children are permanantly pushing boundaries, not to mention buttons.
Will go out to van and have alook measure tomorrow to see what can be made on the cover front.