My db used to sleepwalk. I think he almost made it out of the house on a number of occasions.
By far the most frightening was when we were staying at a hotel once. Mum and Dad had gone down to the bar leaving us in the room with a monitor and with regular checks being made by the childcare service (as you did in the 70's) (I think we were about 9 and 10).
Db managed to climb out of the bathroom window and was discovered standing on the windowsill leaning out towards a drainpipe as if to try and climb down it!
He just grew out of the walking but also used to suffer from night terrors which were scary.
Could you use a gate at the top of the stairs if you are worried?
You've probably googled it anyway, but here