Hi,
I don't think it can be achieved in standard SAP. You can use the help of your technical team to write a program to read the project planned cost and update it to the sub-projects..
You would need to look at tables RPSCO, PROJ, PRPS.
Also how do you want the sub-projects be updated? Is business looking at real time updates once they do cost planning of the actual project.. You need to get clarity from business on how the data needs to be updated.
hope it helps..
Regards
Sammar