Font size

In the stylesheet. You may want to google for styling nested lists.
(hint: if in your style there is something about the lists in the sidebar, like
#sidebar ul li { font... etc}
you will need to add new declarations for every nested list:
ul li li
and
ul li li li li...

Thanks for your answer, it says:

#sidebar {
float: left;
margin-top: 20px;}

How can I add the font size to the children categories?

Insert the style into the stylesheet as the example below

#sidebar ul{
font-size: 14px;
}

#sidebar li{
font-size: 12px;
}