Thank you for writing.
You could add descriptor items by hand if you set up the control in unbound mode: http://docs.telerik.com/devtools/winforms/datafilter/working-with-descriptor-items/unbound-mode
. However, you should be careful when applying such a filter is it may contain fields from both of your master and child tables.
I can suggest using separate data filter controls for the different tables. If that is not suitable you can use the same RadDataFilter
and bind to the master or child tables when you need to. This way your expression would always be valid as the control would contain fields from only one table.
I hope this helps. Please let me know if you need further assistance.