I need help finding the right strategy

Following figure is a part of my private project.

Next is my classes..

What I'm trying to make is a list of several objects as elements. This list can have other lists as elements, and the objects that can be thought of as leaf nodes are Mpml objects.

// example
[
    1 => Mpml object, // Mpml is leaf object.
    2 => Exam object, // The second object have some other childrens.
    3 => Mpml object
]

The second diagram is the result of thinking about how to implement it. But no matter how much I think, it feels still something wrong. HELP!!

I'm using Laravel 5.8 and have a very basic knowledge of design pattern(like, knowing what it is but cannot easily implement it).