Make Tracks Or Paths Of Parent Has Many Child

I have a  tblActivity table :

ActivityID       ActivityName-----------------------------1                A12                A23                A34                A45                A56                A67                A78                A89                A910               A10

and related it in tblParentActivity table with parent child relationship as:

ParentActivityID     ChildActivityID-------------------------------------12132425364757687989910

I want to write a SQL query which will make it in tracks depend on how many childActivity in one parentActivity like :

ActivityID    TrackGroup-------------------------112141719110112225272921021333638393103

using sql sever 2008 r