Queries :: Sequence Number Reset At Each ID Exchange

I have two tables - one with parent Records, the other with child records. The ID links the two tables .I want to add a sequence number on the child table which resets at each ID change. What would be the best way to accomplish this?

Code:
PC_ID ID Name Address

P 1 Parent1 Address1
P 2 Parent2 Address1

PC_ID ID Name Address SEQ

C 1 Child1 Address1 1
C 1 Child2 Address1 2
C 1 Child3 Address1 3
C 1 Child3 Address1 4
C 2 Child1 Address1 1
C 2 Child2 Address1 2
C 2 Child3 Address1 3