Hi Gert,
To conditionally hide the plusminus button when there in no child records to showup, the best approach would be is to have a customengine for your GridGroupingControl. Please refer the following Knowledge Base(KB) article for implementing this task.
http://www.syncfusion.com/support/kb/grid/Default.aspx?ToDo=view&questId=458Here is the sample download link for your reference
http://www.syncfusion.com/support/user/uploads/ggc_hideemptychildtables_e194a3d8.zipLet us know if you have any other questions.
Have a nice time.
Regards,
Rajagopal