box = gtk_box_new (GTK_ORIENTATION_VERTICAL, 6);
gtk_box_pack_start (GTK_BOX (box), child, FALSE, FALSE, 12);
This can be done with GtkGrid as follows:
grid = gtk_grid_new ();
gtk_grid_set_row_spacing (GTK_GRID (grid), 6);
g_object_set (child, "margin", 12, NULL);
gtk_grid_attach (GTK_GRID (box), child, 0, 0, 1, 1);