Sad to say, Anna, it sounds like you need a Special Resolution to be approved at a General Meeting with not more than 25% of those voting at the meeting, either present or via proxies, to be in favour. Which means you do, indeed, need a Special By-Law; professionally created, too, probably.
As nice and sensible as it sounds, perhaps not all residents, be they owners or otherwise, might like their peaceful vista turned into a kids play area and I hope you can see the possibility for that to be a legitimate point of view.
But why not take a friendly ‘straw poll’ right now to see how sentiment lies? Before you waste too much time and emotional energy.