To export the call log for all of your sub-accounts, go to Activities ▸ Export Log.
In the Export Calls screen, enter the email then scroll down to the Filters section, Toggle the box Export all calls for all sub-accounts.
In the Fields section, select Account Name and Account ID along with any other fields to include in the export.
Click Build Report.
An email will be sent with the call log from every sub-account and the spreadsheet can then be sorted by Account ID or Account Name.