Hi Stephane,
Thanks for your interest in Syncfusion Products.
To tab through the cells across parent-child grids, TableControlMoveCurrentCellDirection event can be handled. In this event, the movement of currentcell focus can be handled and set accordingly. Please refer to the following sample in which this has been implemented.
http://files.syncfusion.com/support/samples/Grid.Windows/7.3.0.20/F53455.zipRegards,
Rajadurai