Usually there is no requirement to bring a building up to current building standards except if new work is being done. Eg. if the balcony rails were being replaced, then the replacements would have to comply with current standards.
On the face of it, your position is reasonable, and the committee’s position is unreasonable if there are much more major balcony modifications. On the other hand, perhaps those would also not be approved and perhaps never were. Perhaps it is only because you did the right thing by asking for permission that you have found yourself in a position where the committee feels it has to say no.
I would suggest that a reasonable compromise would be to permit the attachment of something temporary and reversible that provides the added safety you want. The committee could give you permission for that with a time limit (when your children reach some particular age, for example). Then, with your immediate concern addressed, you could work on a longer term term solution. What about those enclosed balconies? Were they permitted? Does the OC want to take action on all modified balconies? Alternatively, does it want to have a period of discussion about what modifications should be permissible? Will all the balcony rails need replacing sometime just due to age, wear and tear? If so, what style should the building decide to install everywhere (and of course, compliant with the current standard)?