Hi Nabia,
Thank you for your interest in Syncfusion Products.
Please refer the below code snippet to hide the column in child table.
[C#]
GridTableDescriptor childTableDescriptor = (GridTableDescriptor)GridGroupingControl1.TableDescriptor.Relations[0].ChildTableDescriptor;
childTableDescriptor.VisibleColumns.Remove("childID"); // childID, ChildTable column name.
Also please refer the sample below which illustrates the above.
http://websamples.syncfusion.com/samples/Grid.Web/7.1.0.21/F79281_HideChildTableColumn/main.htmPlease let me know if this helps you.
Thanks,
Rekha