My first thought on reading this was liability. If you were to set aside an area for children to use as a play area you would have to review your insurance to ensure you were covered, and it would likely increase your premium. Also you would probably have to check with the Council. It seems ridiculous that you have to be so cautious, but unfortunately I think if you are designating something as for use by children, that is what you have to do.
I differ from Jimmy in that I think if a common property area is to be allocated for a specific use, then it most probably would require a special resolution, because effectively you are preventing others from using it for other purposes. For those who don’t have children it would be reasonable for them to say they are being deprived of the use of that area, or that others are benefitting in a way they are not.
That is ironic because as Anna says, the area is little used. I used to live in a block of flats where I put up a resolution to grant exclusive use of the top of some back storage areas, which effectively were balconies coming off the windows of two lots. The other owners said no way would they allow that – which was really stupid because no-one could access those two spaces except the respective owners.
It is a nice idea, so rather than designating it for children why don’t you put up a suggestion for landscaping and putting in the fences, as a general improvement and amenity for everyone. Surely no-one would object to that, and that would only need to go to the EC.