Even if you are together at home, you don’t always have your eyes on him. Specifically, when they are a little older around the age of 5 and up. They might be playing on the balcony or by the pool and you’re not there.
Have a perimeter sensor system that omits a low but loud disturbing or irritating sound when someone gets close. Nothing to frighten them, just let them know there is danger close by and they should stay away from the edge of the balcony or the pool.