How to add a Toggle Main menu catagory.

Hi,

on the page in the link provided, just for that particular menu category (brands), I have a lot of options for people. I don’t want these all to show up on the side of the page as its taking up so much space. For every other menu catagory (eg, ‘Starter Kits,’ ‘Mods and devices,’ ‘tanks,’) I am happy with the checkbox option, but for this main menu category called ‘Brands’ it just too long. I’d like to add a closed toggle option so it doesn’t take up so much of the side of the page.
If you provide me code, please tell me where to insert it?

Kind regards
Leah

The page I need help with: [log in to see the link]