Well, during persist the Parent entity is “new” and Hibernate ORM just passes that new entity to the validation library before emitting the insert statement. If you update the entity though, Hibernate ORM will only pass objects to the validation library if they are considered dirty. If you want validation to happen even when you just change a collection, you will have to mutate the entity somehow.
You can for example nullify some field (e.g. lastUpdatedTimestamp) which will be updated automatically in a pre update listener.