You can use (what I call) a blank update to force database rules to run on an object:

    trigger CreditRequestTrigger on Credit_Request__c(after update)
    {
        update [select ID from Credit_Lines__c where Credit_Request__c in :Trigger.New];
    }

As Derek F pointed out in comments, you should be careful when updating another object in a trigger in general, as you can cause recursion between the two objects if they can update each other.  This is very easy to do with Master-Detail records, since roll-up fields cause an update to the parent, so if you update something in `Credit_Lines__c` which is part of a roll-up field on `Credit_Request__c`, you will end up calling your `Credit_Request__c` trigger again.  You can prevent this using static variables, which will retain their value throughout the entire database transaction.