Mo. Rev. Stat. § 306.220: Personal flotation device, who must wear, when, exception, penalty.

Each person under the age of seven who is on board any watercraft which is on the waters of this state shall wear a personal flotation device which is approved by the United States Coast Guard. Any person who allows a person under the age of seven to be on board any watercraft which is on the waters of this state without wearing a personal flotation device shall be deemed guilty of a class C misdemeanor. This section does not apply when the person under the age of seven is in a part of a watercraft which is fully enclosed, where such enclosure will prevent such person from falling out of or being thrown from the watercraft.