Looking at this, http://www.iccube.com/livedemo/?ic3reportName=I%20want%20a%20BigMac
I want to do the same thing but with a multi-selection (Buttons) first filter which would reduce the choices list on a second filter (TreeFilter) Thanks.
In that report's case if the first filter is a multi-select one, you have to add this mdx in the second's Query Wizard: Descendants(@{category}).
UPDATE:
If you want to return just the first level children, you have to add a parameter for the level, like this: Descendants(@{category}, 1). Without the second parameter near the children it returns also the parent. Also you can check here how this function works.