Why is this sentence correct if you can't have "would" for repeated action in the past with a state verb?

Because "have" is not a state verb in that sentence. The grandmother doesn't just possess a snack- she prepares it and gives it to the grandchild. There are many usages of "have" in English. 

"Have" is not a state verb. It is a transitive verb. She had a surprise. 

I've never heard of your rule that you can't use "would" for repeated action in the past with a state verb. "Grandma would always be in the kitchen when we arrived." I don't think anyone would object to that sentence other than the person who made up that "rule" for no reason. 

I do not know what you mean by a "state verb".  A verb is a verb, whatever its function in the sentence.

Your example "I am having a surprise for you." is meaningless.  Likewise "I would have short hair when I was younger." is rubbish.  "I HAD short hair when I was younger" is the correct and ONLY form.  Your division of verbs into "action" or "state" form is nonsense.

It is correct. 'would have' also implies that she made a special effort to make sure there was a nice surprise ready.

It is correct but if you don't like it, you could correctly say, "Our grandmother always had a little surprise waiting for us when we visited" since it conveys the the same thing in the past tense