Surrey Police said Sunday that the eighth child was kept in overnight for observation "but his injuries are not thought to be significant."
The Health and Safety Executive is investigating the accident, which happened as thousands gathered Saturday to watch Bonfire Night fireworks in Woking, London.
Journalist Andy Datson said he saw up to 40 children playing on the slide, which he estimated was about 30 feet high and "looked pretty flimsy."
Communities across Britain hold early November fireworks displays to commemorate Guy Fawkes' failed 1605 plot to blow up Parliament.