public string ChildNodesPath { get; set; }

Public Property ChildNodesPath As String

<dxg:TreeViewControl ItemsSource="{Binding EmployeeDepartments}" 
                     ChildNodesPath="Employees" 
                     TreeViewFieldName="Name"
                     HasChildNodesPath="HasChildNodes"/>

using System.Windows;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using DevExpress.Mvvm;

namespace TreeViewChildNodesSelector {
    public class MainWindowViewModel : ViewModelBase {
        public MainWindowViewModel() {
            EmployeeDepartments = Departments.GetDepartments();
        }
        public List<EmployeeDepartment> EmployeeDepartments { get; set; }
    }
    public class Employee {
        public Employee(int id, string name) {
            ID = id;
            Name = name;
        }
        public int ID { get; set; }
        public string Name { get; set; }
    }
    public class EmployeeDepartment {
        public string Name { get; set; }
        public ObservableCollection<Employee> Employees { get; }
        public bool HasChildNodes { get; set; }

        public EmployeeDepartment(string name, IEnumerable<Employee> employees) {
            Name = name;
            Employees = new ObservableCollection<Employee>(employees);
        }
    }
    public static class Departments {
        public static List<EmployeeDepartment> GetDepartments() {
            List<EmployeeDepartment> departments = new List<EmployeeDepartment> {
                new EmployeeDepartment("Management", new Employee[] {
                new Employee(0, "Gregory S. Price")
                }, true),
                new EmployeeDepartment("Marketing", new Employee[] {
                new Employee(1, "Irma R. Marshall"),
                new Employee(2, "Brian C. Cowling"),
                new Employee(3, "Thomas C. Dawson"),
                new Employee(4, "Bryan R. Henderson"),
                }, true),
                new EmployeeDepartment("Operations", new Employee[] {
                new Employee(5, "John C. Powell"),
                new Employee(6, "Harold S. Brandes"),
                new Employee(7, "Jan K. Sisk"),
                new Employee(8, "Sidney L. Holder"),
                }, true),
                new EmployeeDepartment("Production", new Employee[] {
                new Employee(9, "Christian P. Laclair"),
                new Employee(10, "James L. Kelsey"),
                new Employee(11, "Howard M. Carpenter"),
                new Employee(12, "Jennifer T. Tapia"),
                },false),
                new EmployeeDepartment("Finance", new Employee[] {
                new Employee(13, "Karen J. Kelly"),
                new Employee(14, "Judith P. Underhill"),
                new Employee(15, "Russell E. Belton"),
                },false)
            };
            return departments;
        }
    }
}

Imports System.Windows
Imports System.Collections.Generic
Imports System.Collections.ObjectModel
Imports DevExpress.Mvvm

Namespace TreeViewChildNodesSelector
    Public Class MainWindowViewModel
        Inherits ViewModelBase

        Public Sub New()
            EmployeeDepartments = Departments.GetDepartments()
        End Sub

        Public Property EmployeeDepartments As List(Of EmployeeDepartment)
    End Class

    Public Class Employee
        Public Sub New(ByVal id As Integer, ByVal name As String)
            ID = id
            Name = name
        End Sub

        Public Property ID As Integer
        Public Property Name As String
    End Class

    Public Class EmployeeDepartment
        Public Property Name As String
        Public ReadOnly Property Employees As ObservableCollection(Of Employee)
        Public Property HasChildNodes As Boolean

        Public Sub New(ByVal name As String, ByVal employees As IEnumerable(Of Employee))
            Name = name
            Employees = New ObservableCollection(Of Employee)(employees)
        End Sub
    End Class

    Module Departments
        Function GetDepartments() As List(Of EmployeeDepartment)
            Dim departments As List(Of EmployeeDepartment) = New List(Of EmployeeDepartment) From {
                New EmployeeDepartment("Management", New Employee() {New Employee(0, "Gregory S. Price")}, True),
                New EmployeeDepartment("Marketing", New Employee() {New Employee(1, "Irma R. Marshall"), New Employee(2, "Brian C. Cowling"), New Employee(3, "Thomas C. Dawson"), New Employee(4, "Bryan R. Henderson")}, True),
                New EmployeeDepartment("Operations", New Employee() {New Employee(5, "John C. Powell"), New Employee(6, "Harold S. Brandes"), New Employee(7, "Jan K. Sisk"), New Employee(8, "Sidney L. Holder")}, True),
                New EmployeeDepartment("Production", New Employee() {New Employee(9, "Christian P. Laclair"), New Employee(10, "James L. Kelsey"), New Employee(11, "Howard M. Carpenter"), New Employee(12, "Jennifer T. Tapia")}, False),
                New EmployeeDepartment("Finance", New Employee() {New Employee(13, "Karen J. Kelly"), New Employee(14, "Judith P. Underhill"), New Employee(15, "Russell E. Belton")}, False)
            }
            Return departments
        End Function
    End Module
End Namespace

<dxg:TreeViewControl ItemsSource="{Binding EmployeeDepartments}"
                     ChildNodesPath="Employees"
                     TreeViewFieldName="Name"/>