create a public property or a function in masterpage which get ur required data.
On the child aspx source add the following tag:
<%@ MasterType VirtualPath="~/MasterPageName.master" %>
in the codebehind where u need this data simply say something like:
strBlah = Me.Master.<Call property or function> ("Me" = "this" in c#)
u r done.
Hope this helps.