How to get list of matrices?

I tried Configuration.Instance.GetItemsByKind(Kind.Matrix) and I get list of servers. Then I call GetChrildren for the particular server and I get empty collection but I configured 3 matrices on server.

You must use..

Configuration.Instance.GetItemsByKind(Kind.Matrix, ItemHierarchy.SystemDefined)

If you use default or ItemHierarchy.UserDefined you get nothing. (No user defined groups are possible for Matrix items.)