At one house we had a chain up high.
At this house (and our last house) we have a security door, which we lock and I put the key up high, out of sight.
Would your Dad agree to a chain lock or one of those slide locks on the inside of the door (up high)?
ETA: My children developed road sense at different ages (My eldest understood sooner, but my second child took ages). It wasn't something I was able to teach them quickly (much to my frustration!). It kind-of happened over time with lots of reinforcement until suddenly I realised that they understood.