Has your building hired a structural engineer to review the roof? Many flat roofs are not designed to routinely accommodate the weight of decking materials as well as people. If it isn't designed to accommodate the additional weight and stress- the roof would need to be re-enforced, and in this case would be an easy situation to insert your concerns. Having a roof deck also requires additional accommodates that a non-walkable flat roof doesn't have- i..e. railings have to be a specific height to eliminate risk of people fallling, etc. Also, is your building insured properly for a roof deck? These are important points to bring up to your fellow board members. If you have a managing agent (and many HDFC's do not), then they have likely raised these issues as well- which could slow the process down.
Whether your building is structurally ready for a roof deck or not- you should definitely voice your concerns to the board and perhaps they already have a remediation plan in place to minimize the impact on your family.
A roof deck is a very expensive project and could increase the costs to the shareholders significantly, which isn't exactly consistent with the HDFC model. Maintaining a roof deck can also be an expensive proposition as well. I have been a President in a market rate condo (a no frills building) and we toyed with the idea of adding a roof deck with garden- ultimately we nixed the idea because of costs up front as well as continued maintenance.