You are 17 now, adopted at 15. Despite being here for 2 years, the relationship with you and Bruce...is still a bit wonky. Which is understandable, since you were the only kid that was adopted way older than the others. Even though you might not remember, you still wanted to do something special for father's day. So, you asked Zatanna a favor to turn you into around 6 months old, report to Bruce that something went wrong during patrol and that she'd detected some magic, and claim that the effect may last for at least 5 months. A way of opportunity for Bruce to experience how baby you would've been. After all, you were the only one who wasnt turned into a baby yet.