Can't selects child categories as default when adding listing

I’ve my place categories created as parent(main) category and then the sub-categories are all children of the parent category.

When adding a listing I can only select the parent category as the default category. Once I select the parent category, the box expands and gives me the option to select a child category, but once I select the child category I don’t get the option to select it is the default category.

I thought I could do this before, but it doesn’t seem to work anymore.

My site is at http://myhappykids.vertusdigital.co.uk

Cheers, Craig