you can run the subreports is a different section of the footer, above the calculation. If you want everything on one line, and the results of the subreports are more than 1 line, things can be complicated, but if the results are nothing more than the shared variable you can do:
place the subreports in there own section of the band. in the subreports, supress the output fields. In the section that the subreports run, go to the Section Expert and check the option 'Supress blank sections'. This will effectively 'Hide' the subreport and their output. To display the values, create formulas to display the appropriate shared variables and calculation.
HTH
thanks to DBlank for reminding me how to 'suppress' a subreport, I forget every now and then.