There isn't a specific term in the Harry Potter canon for a wizard whose parents are both Squibs. However, we can have some fun speculating:
* A "Paradox Wizard": This could be a humorous term, highlighting the unexpected magical ability in a family without magical lineage.
* A "Hidden Spark": This is more poetic, emphasizing how magic emerged despite the lack of apparent magical heritage.
* A "Magical Anomaly": This term leans towards the scientific, emphasizing the unusual nature of the wizard's existence.
Ultimately, the term depends on the tone you want to convey. It's a fascinating idea to explore, as it challenges the traditional understanding of magical inheritance in the wizarding world.