we have parent server and few primary site servers just want to know what is the  difference between parent and central server hierarchy in SCCM 2007

Central has primaries below it (side by side) These would be parents. Where both parents are in parallel and report to the primary

Child Primary is a primary under a Primary server.  Child reports to parent which reports to the next parent.

https://technet.microsoft.com/en-us/library/bb694028.aspx

Find the section marked :Hierarchy Used in All Roaming Scenario Examples

this will give you a graphica example of a Central, primaries and child primaries