The balcony is about chest height on a normal adult. Low enough to see over easily for anyone of a normal height, high enough to mean you absolutely could not fall accidentally even if you were very tall indeed. I don't think it's the architect's job to assume that there are nutters who would throw a child off a balcony wandering around. This type of incident is rare, thankfully. It is unbelievably horrible but absolutely not the fault of the architect or the Tate.
It doesn't feel like a huge open space. I don't like heights at all but did not find it scary as the barrier feels substantial and high.
Holes in public art are a bit different as you can absolutely assume with reasonable confidence that there are curious children (or even adults) wandering about who might just go 'will my finger fit'.