Let test scores and academic records speak for themselves. Blurting out that your son has a slight learning difficulty could nix your chances of his getting in. On the other hand if he has been diagnosed with dyslexia or ADD or some other learning difficulty, then you need to be applying to a school which has qualified staff and programs in place to address those learning issues. But your son's B grade in mathematics or his lack of prowess in fine arts are not deal breakers at most schools.
2. Don't brag about your child's accomplishments.
An oblique reference to your daughter's field hockey abilities is far better than bragging. After all, the admissions staff can read all about her accomplishments in the application. (You did make sure to include all that sort of thing, didn't you?) On the other hand, if she is indeed a very talented athlete, make sure you have her current coach call the school to discuss that. It will be one more check in the 'plus' column on your child's file.
Asking pointed questions is acceptable, to a point. The interview is your chance to get some answers too. Just be very careful not to come across as too arrogant. Schools look at accepting your child as the beginning of a nice partnership. A partnership of equals. If you come across as the dominant partner, that might not help.
Years ago some schools would be influenced by the offer of a large donation ($100,000 or more usually was considered large) in exchange for their accepting little Roderick Jr. Not any more. Most schools will look askance if you try that sort of thing. Diversity is a big deal in schools these days. Similarly, arriving for an interview in the Bentley, dressed in your Oscar de la Renta outfit probably will be seen as ostentatious and out of tune with the school's egalitarian objectives. Being understated and quiet will be quite acceptable. The school probably has a very clear idea of your circumstances anyway.
5. Don't throw your influence around.
Telling the admissions staffer that the CEO of a Fortune 100 company is prepared to write a letter of recommendation for Roderick Jr. will come across as a bit heavy handed. Let them find the letter in your child's dossier themselves. In line with point 4, schools prize parents who will quietly support the school without drawing attention to themselves.
Finally, make the best impression possible by relaxing and just being yourself.