You'll need to add to the link from the main report to the sub-report so that you can set the values of the date parameters.
1. Create the date params in the main report.
2. Under Change Subreport Links, select your From date parameter in the top left and move it to the top right.
3. On the bottom right, uncheck "Select Data in subreport based on field".
4. On the bottom left, drop down the list of parameters and select the From date parameter that's in the subreport.
5. Repeat steps 2 through 4 for the To date.
Also, be aware that Crystal will not do nested subreports so if the report you're using for your subreport also contains a subreport, Crystal will NOT include that nested subreport in its output.
-Dell