spruce I can see where you are coming from with the belt and a short rope to stop you going backwards, but if you took a sidewards fall I doubt your son or stand off would stop the ladder spin, not that I am preaching H&S, I wouldn't even have considered the safety harness to be honest, I would have just done with no three point of contact unless you count my knee on a rung as the third

if it was a commercial I would insist on the eyebolts being fitted at the bottom so the ladder could be secured
domestic if I put my safety head on either they would need to do the same or they windows get missed