An adjustable child device includes a first seat portion, a second seat portion and a belt engaging element. The second seat portion is foldable relative to the first seat portion between an unfolded position and a folded position and at least partially received in the first seat portion when the second seat portion is located at the folded position. The second seat portion and the first seat portion are located on a horizontal plane to form a seat part when the second seat portion is located at the unfolded position. A positioning protrusion is formed on a lateral side of the seat part. The belt engaging element is installed on a lateral side of the seat part. A positioning recess is formed on the belt engaging element for engaging with the positioning protrusion. The adjustable child device can be folded when it is not in use, which saves storage space.