Children under the age of three should be safely seated on the lap of an adult.
Parents should hold children up to three years of age on their lap.
1) Children under the age of three should be safely seated on the lap of an adult.
「3歳未満のお子様は大人のお膝の上で安全に座らせてください。」
2) Parents should hold children up to three years of age on their lap.
「親御さんは3歳までのお子様をお膝の上で抱いてください。」
「3歳未満のお子様」は
children under the age of three
children under three years old
または「3歳までのお子様」という意味で
children up to three years of age
とも言えます。
「膝の上」」は on one's lap
ご参考になれば幸いです！
日本 
