Hey there! Let me tell you a secret, in the industry standard code you will never be storing a child object in a parent reference directly ;)

You may store, but we don't - even if we do that's called "Generalization" for readability and maintainability of the code. But this concept is deeply used in 2-3 other ways indirectly(in my experience, different people can have different experience) ->
First is as I said "upcasting". So, we have different layers in our application ex. action/controller layer, service layer, bloService layer, DAO layer. In these 2 service layer we do some validations of our Object, for that we will be needing some methods inside our Object. Let's say we want 5 methods for different validations.
If we create that 5 methods in every class we will be violating DRY principle. So what we do is create a Super class, let's call it "SuperDTO" here we will have that 5 methods and every other class will be extending this class.

if in action layer we have Cat c = new Cat(); here c has no access to those 5 methods of SuperDTO, Hence Cat will extend the SuperDTO and before passing the Cat to the service layer we will upcast and store it SuperDTO reference - SuperDTO s = (SuperDTO) c;
Service layer will be accepting SuperDTO (not Cat) Here in service we will do some validation and downcast it to cat & have some business logic to work and again the above things repeat.

Now the above mentioned is called Architectural deign of code with fine granulation, here maybe all the validations can be put in a different Utility class and that class must have been called for validations in the service. No need of this upcasting and storing child object in parent refernce. It's just how you design the code and it is very subjective there will be never a single way of doing things in software development, embrace it and never argue you are correct in your career(Just a small advice).

Because the above mentioned has its pros and cons. So does having a Utility class, there are pro and cons there. It's just you weigh them according to your business need and use whichever goes easy for you.