diff --git a/IPython/html/widgets/widget.py b/IPython/html/widgets/widget.py
index 4c576b387..6db671568 100644
--- a/IPython/html/widgets/widget.py
+++ b/IPython/html/widgets/widget.py
@@ -308,7 +308,7 @@ class Widget(LoggingConfigurable):
         their model ids.  Return value must be JSON-able."""
         if isinstance(x, dict):
             return {k: self._pack_widgets(v) for k, v in x.items()}
-        elif isinstance(x, list):
+        elif isinstance(x, (list, tuple)):
             return [self._pack_widgets(v) for v in x]
         elif isinstance(x, Widget):
             return x.model_id
@@ -322,7 +322,7 @@ class Widget(LoggingConfigurable):
         their model ids."""
         if isinstance(x, dict):
             return {k: self._unpack_widgets(v) for k, v in x.items()}
-        elif isinstance(x, list):
+        elif isinstance(x, (list, tuple)):
             return [self._unpack_widgets(v) for v in x]
         elif isinstance(x, string_types):
             return x if x not in Widget.widgets else Widget.widgets[x]
@@ -412,7 +412,7 @@ class DOMWidget(Widget):
             be added to.
         """
         class_list = class_names
-        if isinstance(class_list, list):
+        if isinstance(class_list, (list, tuple)):
             class_list = ' '.join(class_list)
 
         self.send({
@@ -433,7 +433,7 @@ class DOMWidget(Widget):
             be removed from.
         """
         class_list = class_names
-        if isinstance(class_list, list):
+        if isinstance(class_list, (list, tuple)):
             class_list = ' '.join(class_list)
 
         self.send({
diff --git a/IPython/html/widgets/widget_bool.py b/IPython/html/widgets/widget_bool.py
index cce6b9811..211ba312c 100644
--- a/IPython/html/widgets/widget_bool.py
+++ b/IPython/html/widgets/widget_bool.py
@@ -14,7 +14,7 @@ Represents a boolean using a widget.
 # Imports
 #-----------------------------------------------------------------------------
 from .widget import DOMWidget
-from IPython.utils.traitlets import Unicode, Bool, List
+from IPython.utils.traitlets import Unicode, Bool
 
 #-----------------------------------------------------------------------------
 # Classes
diff --git a/IPython/html/widgets/widget_container.py b/IPython/html/widgets/widget_container.py
index ddfccd560..ccbe4d8b2 100644
--- a/IPython/html/widgets/widget_container.py
+++ b/IPython/html/widgets/widget_container.py
@@ -14,18 +14,20 @@ Represents a container that can be used to group other widgets.
 # Imports
 #-----------------------------------------------------------------------------
 from .widget import DOMWidget
-from IPython.utils.traitlets import Unicode, Bool, List, Instance
+from IPython.utils.traitlets import Unicode, Tuple, TraitError
 
 #-----------------------------------------------------------------------------
 # Classes
 #-----------------------------------------------------------------------------
+
 class ContainerWidget(DOMWidget):
     _view_name = Unicode('ContainerView', sync=True)
 
-    # Keys, all private and managed by helper methods.  Flexible box model
-    # classes...
-    children = List(Instance(DOMWidget))
-    _children = List(Instance(DOMWidget), sync=True)
+    # Child widgets in the container.
+    # Using a tuple here to force reassignment to update the list.
+    # When a proper notifying-list trait exists, that is what should be used here.
+    children = Tuple()
+    _children = Tuple(sync=True)
 
     def _children_changed(self, name, old, new):
         """Validate children list.
@@ -36,7 +38,7 @@ class ContainerWidget(DOMWidget):
             http://www.peterbe.com/plog/uniqifiers-benchmark
         which provides the inspiration for using this implementation.  Below
         I've implemented the `f5` algorithm using Python comprehensions."""
-        if new is not None and isinstance(new, list):
+        if new is not None:
             seen = {}
             def add_item(i):
                 seen[i.model_id] = True
diff --git a/IPython/html/widgets/widget_float.py b/IPython/html/widgets/widget_float.py
index ec1868e9a..cad915eb0 100644
--- a/IPython/html/widgets/widget_float.py
+++ b/IPython/html/widgets/widget_float.py
@@ -14,7 +14,7 @@ Represents an unbounded float using a widget.
 # Imports
 #-----------------------------------------------------------------------------
 from .widget import DOMWidget
-from IPython.utils.traitlets import Unicode, CFloat, Bool, List, Enum
+from IPython.utils.traitlets import Unicode, CFloat, Bool, Enum
 
 #-----------------------------------------------------------------------------
 # Classes
diff --git a/IPython/html/widgets/widget_int.py b/IPython/html/widgets/widget_int.py
index 5a706bf5e..4c9aa2c0d 100644
--- a/IPython/html/widgets/widget_int.py
+++ b/IPython/html/widgets/widget_int.py
@@ -14,7 +14,7 @@ Represents an unbounded int using a widget.
 # Imports
 #-----------------------------------------------------------------------------
 from .widget import DOMWidget
-from IPython.utils.traitlets import Unicode, CInt, Bool, List, Enum
+from IPython.utils.traitlets import Unicode, CInt, Bool, Enum
 
 #-----------------------------------------------------------------------------
 # Classes
diff --git a/IPython/html/widgets/widget_selectioncontainer.py b/IPython/html/widgets/widget_selectioncontainer.py
index 0f9156ab8..ef9155992 100644
--- a/IPython/html/widgets/widget_selectioncontainer.py
+++ b/IPython/html/widgets/widget_selectioncontainer.py
@@ -15,7 +15,7 @@ pages.
 # Imports
 #-----------------------------------------------------------------------------
 from .widget_container import ContainerWidget
-from IPython.utils.traitlets import Unicode, Dict, CInt, List, Instance
+from IPython.utils.traitlets import Unicode, Dict, CInt
 
 #-----------------------------------------------------------------------------
 # Classes
diff --git a/IPython/html/widgets/widget_string.py b/IPython/html/widgets/widget_string.py
index e3633365d..e3505c5b4 100644
--- a/IPython/html/widgets/widget_string.py
+++ b/IPython/html/widgets/widget_string.py
@@ -14,7 +14,7 @@ Represents a unicode string using a widget.
 # Imports
 #-----------------------------------------------------------------------------
 from .widget import DOMWidget, CallbackDispatcher
-from IPython.utils.traitlets import Unicode, Bool, List
+from IPython.utils.traitlets import Unicode, Bool
 
 #-----------------------------------------------------------------------------
 # Classes