As long as you don’t personally attack the admissions officers, or write a 4 page screed on the inferiority of the school, you have nothing to worry about. I’ve worked in boarding schools over a long career and have seen “behind the curtain” on this process for a long time.
All of the admissions officers I have known are emotionally functional adults. If your child is offered admission to a school, it’s because the admissions office thinks your kid is great and hopes they’ll join the community and thrive. Yield exists because admissions officials understand that not every family that is offered admission will accept. Ultimately, they want your kid to be happy and well-placed academically and socially. They don’t take declines of enrollment personally.
I have heard so many Admissions Officers make comments along the lines of “Aw, rats! We really hoped that family would choose us because we loved their kid, but in the end, they picked School X.” Then they shrug and pass the salad.
I’m in charge of a popular sport, so I’ve been involved with many yield efforts at the schools I’ve worked at. When we “lose” a kid to a “rival” school, we wish them well and move on. I cannot imagine the folks I’ve worked with feeling “burned” or being more likely to reject a younger sibling in the future. Kids in a family can be quite different, and they need different things. Good educators understand this.
It’s all going to be fine!