There is a AutoRefresh option on the report properties which you set the rate in seconds when rendered in HTML.

I have never tried it so can't comment if it does do the trick and if you can set it only for a sub report.

http://social.msdn.microsoft.com/Forums/sqlserver/en-US/34a94a3d-46fb-4001-a0e7-ca37840e4ef8/ssrs-report-auto-refresh