I would say "I understand why you don't necessarily believe me but why don't you trust native speakers' answers? Are you just asking the same question over and over again for fun?"

Student or Learner
Hi.
Context: Tia's learning English in a language school. Her teacher (a teacher of English, a Chinese) taught her "in the morning of" is a set phrase with a question, the question goes like "Children wake up very early ____ (on/in) the morning of Christmas Day". I then told her the answer is "on" and I also sent her some answers from native speakers about this question. But she was still asking her friends. Chris is one of her friends and today Chris asked me this question. I was annoyed not because Tia had already asked me but she kept asking the question when the answer is obvious and most importantly, she doesn't believe what the native speakers say. I was offended because I spent time asking native speakers and she doesn't believe them. I said to her today:
It's okay if you don't trust me. * But I don't see why you find answers from native speakers wrong. Is it fun keep asking the same question again and again with different people?
*: (I also told her the answer is "on". It's okay she doesn't believe me.)
Please help me to correct my sentences and make it natural.
I would say "I understand why you don't necessarily believe me but why don't you trust native speakers' answers? Are you just asking the same question over and over again for fun?"
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
1- That teacher is wrong about "in the morning of" being a set phrase. Both "in the morning" and "on the morning" are possible, but they mean different things.
When we say "in the morning", we are talking about the time of day.
I feel most active in the morning.
In the morning, birds sing and forage for food.
When we say "on the morning of", we are talking about a day (specifically the morning part of it).
The suspect was spotted lurking in the nearby woods on the morning of 12 April.
He claims he was in bed on the morning of the shooting.
2- I find the sentence in question rather awkwardly phrased. I would say:
"Children wake up very early on Christmas Day/morning".
or
"Children wake up very early in the morning on Christmas Day".
Having said that, "on the morning of Christmas Day" is possible, but I can't see myself using it.
3- Refer to emsr2d2's suggested answer to your main question.
I'd say: Children wake up very early Christmas morning.
But I'll tell you right now: You won't win this one. We have students here who disagree with all of us, every time. It's fun to spar, but some students would rather argue than learn.
I'm not a teacher. I speak American English. I've tutored writing at the University of Southern Maine and have done a good deal of copy editing and writing, occasionally for publication.