It's not exactly the same situation, but my wife passed away last winter and my daughter (8 years) and I have continued to travel since then to various different countries in North America, Europe and the Middle East. I carry a copy of the death certificate with me just in case, but have never been asked to produce it. The questions I've received at border control checkpoints include "Where is her mother?", "Does her mother know of your plans?" but then also about half of the time I haven't been asked any questions at all, rather, the officer directly asks my daughter some personal information, such as her date of birth or her Dad's name (me), or where's home. I have never been asked anything by an airline representative.