A month ago, my husband and I went away for a weekend break. The hotel we were staying in had organised an Easter egg hunt for the excited junior guests, and watching the delight on one little girl's face caused a familiar, terrible cascade of emotion. A torrent of grief and regret, which I struggled to hide. It was the little girl's age that provoked it. She was seven, as I discovered when she told the receptionist while we waited to check in behind her family. The same age my daughter would be now — had I not had the...