A second issue that I should have addressed above: under the model by-laws that apply to thousands of schemes, there is an exception to the general rule that people cannot screw or nail things on to common property without OC written permission – they can screw or nail “any structure or device to prevent harm to children.” It cannot compromise fire safety and it must be “in keeping with the appearance” of the building.
However, presumably “in keeping with the appearance” of the building means that you can't install bright pink screens on a dark brick building. It cannot be blanket ban on installing any screens. People are entitled to install screens for keeping out animals and insects so screens or nets per se cannot be a problem.
The upshot is that arguably, people do not need their OC's permission to install a screen or net for the safety of children, if the model by-laws apply.
As the OC is potentially liable for children falling from common property, the rest of the building should be grateful that individual lot owers are reducing their risk of liability by installing safety devices for kids.