Whip cream until almost stiff, fold in Nutella.
Place two waffles on a plate. Spread 1/4 of the mousse and banana slices on top, repeat the process until all of the waffles have been used. Top with the remaining mousse and banana slices, refrigerate for at least 1 hr.
Pour the icing over the waffle gateau and decorate with Smarties and sprinkles.