Best way to reflect child property to parent

I have some legacy code to be refactored. At first, I have a child class interface: IChild. and some classes that implements that interface, like: ChildType1 : IChild, ChildType2 : IChild and ChildType3 : IChild.

The parent has the child types properties, and each time only one of them get initialized example if ChildType1 has value then ChildType2 and ChildType3 are null:

class Parent
{
    public ChildType1 Type1 { get; set; }
    public ChildType2 Type2 { get; set; }
    public ChildType3 Type3 { get; set; }
    //...
}

The IChild holds some important property Enum:

public interface IChild
{
    ImportantEnum MyEnum { get; set; } 
    //
}

My question: regarding the fact I do not know which one of the Types are initiated, what is the best way to make the child MyEnum property to be available through the parent api?

I'm open for refactoring.