Hi! I am all new to SilverStripe. I am learning SilverStripe because I really like the CMS. I am starting my own company and I want my site to be SS based. I really can use your help. If if have something like this:

<% control Menu(1) %>
// ul - li etc //
<% control Children %>
// ul - li etc //
<% end_control %>
<% end_control %>

The children just won't disappear. They are visible no matter what parent item is active. Now I can use jQuery or some PHP to make them successfully disappear, but I rather have SilverStripe to do so. I don't actually need this for my own site, but I just want to learn SS the best way possible, so that my own site will be as clean and fast as possible. Thank you so much!

What you want is to check if the current page is the page or a descendant of the menu item using LinkOrSection:

<% control Menu(1) %>
	// ul - li etc //
	<% if LinkOrSection = section %>
	<% control Children %>
	// ul - li etc //
	<% end_control %>
	<% end_if %>
<% end_control %> 

http://doc.silverstripe.org/sapphire/en/reference/built-in-page-controls#and-lt-if-linkorsection-section-and-gt