Mo. Rev. Stat. § 306.220

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.