"Table of Contents" web part - How to have expand/collapse buttons? (I can use only CSS/JavaScript/JQuery)

Hi there,

I have a Table of Contents web part which is great. Is there a way so it shows only top level sites and then one can expand or collapse any top level site to see/hide the subsites?

As of now it shows

HR Parent site
- HR sub site 1
- HR sub site 2
IT Parent site
- IT sub site 1
- IT sub site 2

What I want is 

+ HR Parent site
+ IT Parent site
So one can expand or collapse on demand.

Any relevant JS/CSS/JQuery code?

Thank you so much.