[Vocabulary] with extras "to grow in", or "for good measure"

[englishhobby]

In a textbook edited by Arakin (a Russian editor) I found a text about Usa customs and traditions. I don't understand the following sentence completely:
Children's birthdays almost invariably are the occasion for spanking — one spank for each year, with extras "to grow in", or "for good measure".

What I do understand is that in the USA there is a custom of 'punishing' the person who has a birthday in a joking manner. But I can't make out what is meant by extras "to grow in", or "for good measure". Could you please help?

 

[GoesStation]

For good measure is a fixed phrase meaning "just to make sure". To grow in means nothing.

I remember noticing about twenty years ago that this tradition seemed to have died out, at least in my town.

 

[SoothingDave]

If your brother or sister decided to give you playful (or not so playful) punches for your birthday, they would usually give you one for each year and then "one to grow on."

Then maybe add "a pinch to grow an inch" and "a punch to grow a bunch."