It would have to be Gaara. The ability to use sand is unique to shukaku, however gaara maintained it along with some of shukaku's chakra. Iron sand is a form of magnetism release, and is unique to the Kazekage. This means, if he is a puppet user, and an iron sand user, he is most likely the son of kankuro. This is because he would have the ability to use normal sand if he was gaara's child, as it is proven that a small amount of bijuu chakra is passed down from a jinchuuriki. As Gaara had the ability to use sand and not iron sand, we can assume that because iron sand is a KG, and that only one parent with the ability is required to pass down the trait, it must be dominant. This means that Gaara artificially received the sand user trait from shukaku. If the standard sand ability was recessive, Gaara would be unable to use it, due to not having two copies of the allele. However, because this is a bijuu ability, I can assume that it pretty much overwrites the dominant allele of iron sand usage entirely, or rather the magnetism release allele.
Through the power of genetics, we can assume that:
Gaara: No kids revealed
Kankuro: Father of Shinki