Taking into account possible delays with childs walking speed and drink/food pause, here's a suggestion :
Take subway from Gare du Nord to Etienne Marcel. Although it is a
lively and populous area, it's the part that I would remove from the
walking path in order to have more time on other parts.
Walk to the Centre Pompidou, where depending on time of day you could
see various street artist showing besides an outside view of the
museum itself.