How to update Child BC records on update of Parent BC record?

Siebel provides various ways of achieving the same solution, but you need to do the analysis beforehand to pick the right one. Recently I got one basic requirement that:

As soon as the Opportunity Sales Stage gets "Approved", all the Activities (Child of Opportunity) status should automatically set to "Approved".

Any Siebel Geek can offer multiple ways to achieve this solution:
1. Write a small