If you new in MDX you need this book
http://www.amazon.com/MDX-Solutions-Microsoft-Analysis-Services/dp/0471748080/ref=sr_1_1?ie=UTF8&s=books&qid=1228303173&sr=1-1
WITH
MEMBER [Measures].[Top Product in Children] AS
TopCount (
[Product].[ByCategory].CurrentMember.Children,
1,
[Measures].[Unit Sales]
).Item(0).Item(0).Name
MEMBER [Measures].[Top Product’s Units] AS
( [Measures].[Unit Sales],
TopCount (
[Product].[ByCategory].CurrentMember.Children,
1,
[Measures].[Unit Sales]
).Item(0) // in Essbase, need...