A large billboard collapsed at Government House on Saturday, injuring three people including two children visiting on Children's Day.
The accident inside the Government House compound knocked a mother unconscious and left one of her sons with a back injury and the other with a head injury. All three were taken to nearby Mission Hospital.
The injured were identified as Chutima Pleanwong, 36, and her two sons, Jessadaporn Khomsing, 10, and Anucha Khomsing, 7.
Witnesses said strong wind caused the accident while the three were having a meal in the area.
Organisers of the event have agreed to compensate the family.
A large crowd turned up at the Children's Day event held at Government House.
Visiting children waited in a long line to take turns sitting on the prime minister's chair and getting their photographs taken as souvenirs.
A two-year-old boy who was in the queue with his parents for more than an hour became exhausted and fell asleep on the chair during his turn. The sight amused other parents and their children.