Further to Whale’s post, there is also a school of thought that installations such as these would be covered by the model by-law 5 whereby an owner can drive nails or hooks etc into the inside of common property walls (for instance) for the purpose of installing insect screens or in this case a child safety device. Of course this would need to be in keeping with the look of the building but a dark coloured netting can appear to be almost invisible from the outside so may not detract from the look of the building.
The changes to the regulations noted by Whale relate to new guidelines for window openings so I would not consider this relevant in this case.