On the flip side of this my partner and I recently decided to pay the additional fee to Spirit to ensure that us, our 6 year old, and our 18 month old (traveling on lap) could sit together. We paid extra to sit toward the front of the plane to minimize time on the plane after landing. As we were boarding we were told that we had been moved because the row we were in was the exit row and we could not sit there with a child (though they let us select that row no problem when they wanted more money). They reassigned us to the very last row on the plane with no window for our 6 year old and no refund on our seat selection fee. I will probably just take my chances asking at the gate next time.