The most viable suit would be against parade organizers and town for lack of barriers.
Though anybody can be sued for anything, the town itself also enjoys sovereign immunity. As a result, they do not have to comply with any judgement.
Towns, however, can agree to pay settlements for a variety of reasons. That might include cases stemming from this attack.
At the same time, offering settlements may set a bad precident as victims of any crime could then expect settlements from the city as the city could always have done more to prevent crime "X".