$theme.children item=child

Hi all,
following situation:

I use the matrix theme. I have 3 album levels. I want to show all sub-albums(children) of the first level on all pages. Is it possible?

In order to show the sub-albums (children) of the current album you use $theme.children item=child. There must be a way of defining the first album (top level) like $theme.toplevel.children or so. I don't know. I've searched in the manuals but unfortunately I couldn't find anything.

I hope there is some way to solve this problem.

greetings, mansen