But I think there must be a misunderstanding, as it would be unusual for a place near suburbia to be a national park or similar area with restrictions like this.
There's nothing there I can see in the NT park rules that precludes digging or building a fort.
Around the world some place back onto national parks, other places have large areas of undeveloped land that aren't national parks (cattle stations, community land, etc).
The video I linked is representative of the kind of ocean land I grew up on and still live near part time - it's a good place to raise children, plenty to do.
But I think there must be a misunderstanding, as it would be unusual for a place near suburbia to be a national park or similar area with restrictions like this.