Removing a child category after deleting parent?

Hi, I’m a shiny newbie to WordPress so do bear with me…I’ve made my first ‘big’ mistake and was adding categories yesterday when I deleted a parent which had child categories. I deleted the parent category thinking it would delete the child categories linked to it which it didn’t and now I can’t remove the child categories…how do I do this? After this happened, I added more categories but these don’t show up on the blog…why is this? I’d really appreciate your help!

Many thanks

Nickie