这里有一个可能的解决方案，我给你的两个类和print方法添加了构造函数，以便快速了解嵌套结构包含什么。

public class Main {
    static private class Child {
        private String id;
        private List<Child> children;

        Child(String id, Child... children) {
            this.id = id;
            this.children = new ArrayList<Child>();
            for (Child child : children)
                this.children.add(child);
        }
        
        void print() {
            print("");
        }
        
        void print(String tab) {
            System.out.println(tab + id);
            for (Child child : children) child.print(tab + "  ");
        }
    }

    private static class CustomChild {
        private String id;
        private CustomChild child;

        CustomChild(String id, CustomChild child) {
            this.id = id;
            this.child = child;
        }

        void print() {
            print("");
        }
        
        void print(String tab) {
            System.out.println(tab + id);
            if (child != null) child.print(tab + "  ");
        }
    }

    private static List<CustomChild> convert(Child node) {
        List<CustomChild> result = new ArrayList<>();
        if (node.children.isEmpty()) {
            result.add(new CustomChild(node.id, null));
        }
        for (Child child : node.children) {
            for (CustomChild newChild : convert(child)) {
                result.add(new CustomChild(node.id, newChild));
            }
        }
        return result;
    }
    
    public static void main(String[] args) {
        Child tree = 
            new Child("1",
                new Child("1_1",
                    new Child("1_1_1"),
                    new Child("1_1_2",
                        new Child("1_1_2_1")
                    )
                ),
                new Child("1_2",
                    new Child("1_2_1",
                        new Child("1_2_1_1")
                    )
                )
            );
        System.out.println("original tree:");
        tree.print();
        System.out.println("converted forest:");
        List<CustomChild> paths = convert(tree);
        for (CustomChild node : paths) {
            node.print();
        } 
    }
}

main函数创建您作为示例给出的输入结构，并调用convert从该结构生成目标路径列表。
这是上面生成的输出：

original tree:
1
  1_1
    1_1_1
    1_1_2
      1_1_2_1
  1_2
    1_2_1
      1_2_1_1
converted forest:
1
  1_1
    1_1_1
1
  1_1
    1_1_2
      1_1_2_1
1
  1_2
    1_2_1
      1_2_1_1