Deletions are easy -- just destroy the child or unmanage it.

Additions are a little more complex, unless you want to add the items at the end. You can either take advantage of the Xt Composite resource XtNinsertPosition or of the XmRowColumn resource XmNpositionIndex. These resources let you specify where in the list of children the new widget should go; as the XmRowColumn dumbly lays out its children in the order that they are stored internally, the result is to "insert" a child in the list.