Hi, I'm writting something very similar to this, in fact the only difference is how Sirius started treating Harry as a baby, basically Askaban and being forced to stay in number 12 has made Sirius a bit insane so when Harry comes from the MoM with the news that Sirius is free and Sirius and Harry can live together Sirius decides to take it upon himself to invent a new tradition about how Godfather's treat their Godson's. Sirius starts to treat Harry as a baby, dressing him up in nappies and baby clothes but Sirius also pleasures Harry sexually. I've not posted it yet but if your interested in it let me know. Sorry it's not identical to what you want for this challenge, I found it after starting to write my Sirius/Harry Daddykink fic.