Some fields can't be edited when the parent record is locked in Approval Process

The related list objects that cannot be edited are the ones that are the detail in a master-detail relationship.  All other related list items can be edited. 

For example: 

An admin has created an approval process for the Opportunity object so that users have to submit Closed Opportunities for approval to their managers. This organization also has a custom object titled "Expenses" that has a master-detail relationship with Opportunities as well as lookup relationships between Opportunities and the custom objects "Projects" and "Metrics."  After Sales Reps close their opportunities and submit them for approval to their managers (and the record becomes locked), they will be able to edit the Projects and Metrics related list items on that record but NOT the Expenses related list because of the master-detail relationship it has with Opportunities. 

Once the record has been approved, all related list items can be edited (if you have the right to edit the record). 


Additional Information:
Set Up an Approval Process
Object Relationships Overview