When you create a Data Extension based on the SendLog template (more info on SendLoggin here) you have to create the Data Extension on each account you want to use it.
However, if you have an Enterprise 2.0 account and you’d like to have one unique SendLog to log all the send activity in one place, there’s a Business Rule you can ask ExactTarget support to enable.
The Send_Logging Business Rule needs to be turned on with a value of ENT._AUTO in the quality field to support Enterprise Send Logging.
Now all you have to do is create the SendLog Data Extension in the Parent Business Unit and you should be good to go!
Take into account that every single send from every sub account will end logged in the Send Log so make sure you are cautious when enabling this feature since you could be inserting a ton of rows into one Data Extension.