You wouldn't have a problem with your daughter wanting to marry your son, I would hope. GOA's website does not mention affinity linking those who share a Godparent. It only mentions the sponsor and the person sponsored not being able to marry.
God-parents. (Godfather, Gr. Nounos; Godmother, Gr. Nouna).Sponsors at Baptism and Chrismation taking the responsibility for the faith and spiritual development of the newly-born Christian. The Orthodox people highly regard the spiritual bond and relationship between godparents and their godchildren, and marriage between them is prohibited. (see affinity).
Affinity. (Gr. Syngeneia). The spiritual relationship existing between an individual and his spouse's relatives, or most especially between godparents and godchildren. The Orthodox Church considers affinity an impediment to marriage.