Okay, thanks rackle for your help so far. I tried the multiply of the line spacing and it worked fine. The UnifiedDim doesn't work, I only get an empty ItemListBox.
And getting the children doesn't work. As I said before: When window gets created, it doesn't have it's children added yet. But I need to access them in order to set some properties of them.
And one more thing about the wiki article, there's a small mistake in the following code:
wfMgr.addFalagardWindowMapping("WindowsLook/CheckListboxItem", "CEGUI/ItemEntry", "WindowsLook/CheckListboxItem", "Falagard/ItemEntry");
must be:
wfMgr.addFalagardWindowMapping("WindowsLook/CheckListboxItem", "CEGUI/CheckListboxItem", "WindowsLook/CheckListboxItem", "Falagard/ItemEntry");
With the upper part, the CheckListboxItem gets ItemEntry as baseclass, which is wrong, as it needs the class CheckListboxItem as baseclass. In this case, it doesn't make a difference, as CheckListboxItem doesn't have member variables and there everything works as it should.