I have just read the thread where most people’s advice was to supervise child and dog, you may have seen it.
It got me thinking.
A few months ago my dd (2yo) was invited to a nursery friends third birthday party.
Invite said we could leave dd if we wanted as they would happily supervise but, of course, we were welcome to stay.
I decided to stay with dd as she’s not the most confident.
At the party there were loads of indoor activities including pass the parcel, musical statues etc. Plus painting, modelling clay, craft etc.
Outside there was a bouncy castle, my dd seemed to prefer this to the other stuff so we were out there for a while.
I’m so glad I did stay with her because she got bored and began to explore the garden.
About 10 feet behind the bouncy castle, which was on a grassy lawn area, was an (at least) 8 foot sheer drop!
Below was another area of the garden which was paved! 
There was a set of steps at the side of the garden which had gates at the top and bottom but there was no barrier (ie fence or something) besides a border with flowers around the ‘top garden’ which luckily my dd wouldn’t go near because ‘dirty’ 
There was one poor frantic mummy though who was following her little boy everywhere for fear he would go over the side. He’d found a football too which didn’t help, it actually went over to the ‘lower level’ and wasn’t retrieved in the end.
I assume birthday girl was used to the garden layout so her mum didn’t see the problem but I was shocked to see such a big drop where little ones play!
Also, of the children who were outside, only their parents appeared to be supervising them, everyone else was inside.
I’m not sure what my aibu is but was it ‘U’ to not ‘childproof’ the garden in these circumstances?