Hawaii state law requires that all children under the age four be restrained in a federally approved child safety seat. The current law requires that children ages four through seven ride in a booster seat or car seat any time they are in a vehicle. The only exceptions are for children taller than 4'9" or those riding in vehicles with lap-only seatbelts in rear seats.
Hawaii's law advises that children who have outgrown harnessed car seats (those who weigh between 40 and 65 pounds) should use a booster seat until they fit an adult seatbelt properly. This typically occurs when a child is at least 4'9" tall and weighs at least 80 pounds.