$(function() {
    $("#gantt").dxGantt({
        validation: {
            autoUpdateParentTasks: true,
            validateDependencies: true
        },
        // ...
    });
});

autoUpdateParentTasks

Specifies whether to recalculate the parent task's duration and progress when its child tasks are modified.

Type:

Boolean

Default Value: false

The Gantt uses the following rules to determine whether to update a parent task when its subtask is modified.

A parent task's duration equals a summary duration of its child tasks.
A parent task and its first child starts at the same time.
A parent task and its last child ends at the same time.
A parent task's progress is a summary progress of its child tasks.

DevExtreme Gantt - Parent and Child Tasks Validation

validateDependencies

Enables task dependencies validation.

Type:

Boolean

Default Value: false

The Gantt allows you to validate relationships between tasks when they are edited.

User Action	Dependency Type	Gantt's Reaction
Move a predecessor task (Task 1) to the left or right.	Finish to Start (FS): Start to Start (SS): Start to Finish (SF): Finish to Finish (FF):	The UI component moves a successor task (Task 2) forward or backward to the same time interval.
Move a successor task (Task 2) to the left.	Finish to Start (FS): Start to Start (SS): Start to Finish (SF): Finish to Finish (FF):	This change is denied as it violates dependency rules. The control displays a popup window and suggests the following actions: Cancel the operation. Delete the dependency.
Move a successor task (Task 2) to the right.	Finish to Start (FS): Start to Start (SS): Start to Finish (SF): Finish to Finish (FF):	The control displays a popup window and suggests the following actions: Cancel the operation. Delete the dependency. Move the task and keep the dependency. It creates a gap between tasks. Note that you can move tasks forward and backward within this gap while it meets the dependency rules.