trigger SubscriptionEmail on Account (after update) {
 
                  for(Account acc: Trigger.new){
                           Account oldAcc = Trigger.oldMap.get(acc.Id);
                            if(oldAcc.Subscribed__c != acc.Subscribed__c && acc.Subscribed__c == true)
                                 { 
                                         List<Contact> children = [ SELECT Id, AccountId, Customer__c from 
                                         Contact where AccountId = :acc.Id];
                                        List<Contact> newids = new List<Contact>();
 
                                        for(Contact con: children){
                                                if(con.Customer__c != acc.Subscribed__c){
                                                        con.Customer__c = acc.Subscribed__c;
                                                newids.add(con);
                                                }
                                        }
                                        if (newids.isEmpty()== false){
                                                update newids;
                                       }
                                }
                 }
  }