Child promotion denied after purchase

FlySafair recently ran a “kids fly free” promotion and I jumped at it because kids aged 2+ pays full price. Our baby turns 2 between the depature and return dates, but when I entered her as “child” (to show the higher price), the system said she’s must be captured as “infant”. I did so and notified FlySafair via email. Apparently I should’ve booked two one ways and must now pay for the full return ticket as a “child” and the discount no longer applies. Please tell me how this is fair?