I guess I'm a big meanie because if a five-year-old doesn't want to eat any of someone else's birthday cake, I would not be providing special consolation extra cake. Five is not too young to learn that everything is not always about you.

For me it depends.

Allergies/food intolerances? They get a small safe alternative dessert

Serious sensory issues? They get a small alternative dessert they can eat

If it's just a dislike/preference, tho, no, no second dessert.