ObservableCollection<Quest> сhildren;

children.CollectionChanged += children_CollectionChanged;

сhildren = new ObservableCollection<Quest>();

varCollectionChanged += children_CollectionChanged;

//после убийства старого и создания нового объекта

children.CollectionChanged = varCollectionChanged;

public ObservableCollection<Item> Children
{
    get { return _children; }
    set
    {
        if (_children != null)
            _children.CollectionChanged -= children_CollectionChanged;

        _children = value;

        if (_children != null)
            _children.CollectionChanged += children_CollectionChanged;
    }
}

ObservableCollection<Item> _children;

сhildren = new ObservableCollection<Quest>();

сhildren.Clear();

сhildren = new ObservableCollection<Quest>();

protected ObservableCollection<Quest> _children;
protected NotifyCollectionChangedEventHandler _children_CollectionChanged;

_children_CollectionChanged = Children_CollectionChanged;

_children = new ObservableCollection<Quest>();
_children.CollectionChanged += _children_CollectionChanged;

Quest quest = quest.DeepClone();

        public static T DeepClone<T>(this T obj)
        {
            using (MemoryStream ms = new MemoryStream())
            {
                BinaryFormatter bf = new BinaryFormatter();
                bf.Serialize(ms, obj);
                ms.Position = 0;
                return (T)bf.Deserialize(ms);
            }
        }

model.Children.Add(new Quest());

model.Children.Add(new Quest());