Even if that were a logical approach to safeguarding, which it isn't, what you are suggesting should happen is not what has happened.
If this situation had been considered on an individual case basis, balancing the needs of the trans student against the needs of others, both the OP's daughter and the OP would have been asked by the school if they were OK with this, in advance of the trip.
Instead, the OP's daughter turned up on a school trip and was told, without any advance warning, that she was to be sharing a room with a child of the opposite sex. And the poor child has been so gaslit that she did not even say anything, not to the school and not even to her own mum. The OP would still be in total ignorance of the fact that her daughter had been put at risk if she hadn't thought to ask what she thought was a pretty inconsequential question. So the OP was kept in the dark about a serious safeguarding issue affecting her daughter.
Nobody except the trans child has been taken into account. There has been no balancing of needs here.
And for what it's worth, any adult who had even the most basic understanding of safeguarding and had not been brainwashed by gender ideology would put two opposite sexed teenagers in the same sleeping accommodation, even if both students asked to be together and their parents were fine with it.