don't validate ContainerWidget.children
don't validate ContainerWidget.children

just use a plain Tuple, so we don't have to define a new Trait

ipython/ipywidgets,SylvainCorlay/ipywidgets,cornhundred/ipywidgets,jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,cornhundred/ipywidgets,cornhundred/ipywidgets,SylvainCorlay/ipywidgets,cornhundred/ipywidgets,jupyter-widgets/ipywidgets,cornhundred/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets