Try using to progressive disclosure to reveal the reminder when they enter the birthday option.
Since the birthday is optional, having the extra 'reminder' control is unnecessary (and potentially confusing) if they won't fill out the control on which it's dependent upon.
Why not take advantage of interaction flow and time, revealing the element only once the user has chosen to enter a birthday? This will reveal the relationship between the birthday and the reminder:
I'm not sure about your use case, but I used a checkbox control to allow them to opt-in. It does mean it's another click for the user, but it allows them to feel like they're giving explicit permission in this case.