@Valid annotation is not validating the list of child objects

You need to decorate addresses member of UserAddressesForm with @Valid annotation. See section 3.1.3 and 3.5.1 of JSR 303: Bean Validation. As I explained in my answer to the question Is there a standard way to enable JSR 303 Bean Validation using annotated method, this is the real use of @Valid annotation as per JSR 303.

Edit
Example code: Hibernate Validator- Object Graph. (The list of passengers in Car)

Edit From Hibernate Validator 6 Reference doc:

In versions prior to 6, Hibernate Validator supported cascaded validation for a subset of container elements and it was implemented at the container level (e.g. you would use @Valid private List<Person> to enable cascaded validation for Person).

This is still supported but is not recommended. Please use container
element level @Valid annotations instead as it is more expressive.

Example:

public class Car {

        private List<@NotNull @Valid Person> passengers = new ArrayList<Person>();

        private Map<@Valid Part, List<@Valid Manufacturer>> partManufacturers = new HashMap<>();

       //...
   }

Also see what’s new in Bean Validation 2.0/Jakarta Bean Validation.