Hide subcategories in right sidebar

Hi all,

first at all I want to apologize for my english, but I hope everyone can understand what I want to achieve. On my blog www.goldsprint.de I have only a right sidebar and the categories are displayed now under "Kategorien". Theire is one categorie "Bikes" that has three subcategories that are displayed there also right know. My goal is that they are only displayed in the menu when the user is klicking on the categorie Bikes. I allready searched for a widget who could do this but only found a drop down menu which i dont want to use. Is something like this possible, I really dont have much knowledge about this stuff and would appreciate some help?

Best regards
Alex