Don’t display child categories

Hi all. I hope this is the correct section to ask.

I am new to WordPress and I’ve been messing with a template for a few days. I am using the Sidebar Widget, but I would like for child-categories not to be displayed on the navigation. I would like to use them to link relevant posts together, but I will very soon end up with loads of child-categories in one particular category, and I just don’t want it to show. Is this possible, and what can I do to achieve this?