Mike,

Is this what you're trying to do?

trigger GenerateCommission on Revenue_Event__c ( after insert )
{
    //  build a set of Accounts (doctors) related to each Revenue Event in this trigger
    Set<Id> set_AccountIDs = new Set<Id>();

    for ( Revenue_Event__c event : Trigger.new )
    {
        set_AccountIDs.add( event.Account__c );
    }

    //  query the Account records (doctors) related to the Revenue Events in this trigger
    Map<Id,Account> map_Doctors = new Map<Id,Account>
    (   [   SELECT  Id, (SELECT Id, Practitioner_Account__c, Sales_Agent__c FROM Commission_Calculations__r WHERE Active__c = true)
            FROM    Account
            WHERE   Id IN :set_AccountIDs
        ]
    );
    if ( map_Doctors.isEmpty() ) return;

    List<Commission__c> list_Commissions = new List<Commission__c>();

    //  for each Revenue Event,
    for ( Revenue_Event__c event : Trigger.new )
    {
        //  look up the associated Account (doctor)
        Account doctor = map_Doctors.get( event.Account__c );

        //  for each Commission Calculation related to the Account (doctor),
        for ( Commission_Calculation__c calculation : doctor.Commission_Calculations__r )
        {
            //  create a new Commission record
            list_Commissions.add
            (   new Commission__c
                (   Revenue_Event__c            = event.Id,
                    Sales_Agent__c              = calculation.Sales_Agent__c,
                    Commission_Percentage__c    = calculation.Id
                )
            );
        }
    }
    //  insert the new Commission records into the database
    insert list_Commissions;
}

If this helps, please mark it as a solution, and give kudos (click on the star) if you think I deserve them. Thanks!

-Glyn Anderson
Certified Salesforce Developer | Certified Salesforce Administrator