Also have a DD with an unconditional offer at C for this year and it's been a steep learning curve.
If applying from a very selective independent, my advice would be to choose a college carefully. Go for one that has a high ratio (80% ish) of applicants and intake from the state sector (Kings springs to mind, or perhaps Homerton) as opposed to one where state school applications are lower (eg St Johns where is more like. Colleges that already meet or exceed the university- wide WP targets (about 75% intake from the maintained sector I think) are more likely to be able to offer to their strong applicants from the independent sector. I know they say college choice shouldn't matter, but colleges do have varying acceptance rates for different sectors and the issue of meeting the WP quotas does come into it, to a greater or lesser extent. Chances of being selected from the pool if applying from an independent are lower than ever, so college choice matters now more than ever.
If applying from the state sector, don't be put off by the "public school" reputations of colleges such as St John's or Trinity as these colleges are possibly still playing catch-up a little in terms of WP targets. So possibly more likely to take a state school applicant, all things being equal.
I only found about all this after DD applied. There is a website called Unifrog that shows success rates across colleges for different sectors. I think only students can log in though (not sure)?
Also, look at the interactive graphs on the Cambridge admissions pages. Look for a college with a good and reasonably consistent acceptance rate for the subject, as far as possible. Also, you may notice that some subjects at certain colleges will routinely take from the winter pool every year, while others virtually never do. Probably better to go for a college that doesn't seem to routinely take from the pool, but tends to offer to its direct applicants.
Try to find out about the interview process for the subject across the colleges because this can vary significantly. Eg, if your DC really wants to talk about their PS, look for a college that has a 'general interview' in the mix (because this is where the PS and wider interests get discussed), rather than 2 academic interviews.
If they are given reading a few days before an interview, they shouldn't only read that, but also read one or two of the texts referenced in it. The interviews are often designed to simulate a tutorial and this is what they would be expected to do for a tutorial.
Other interviews may give reading just before the interview or something during it, in which case, they just need to think on their feet. But look at the interview format, because different DOS have different ideas about the best way to select. Also look at the interests of the DoS (DD didn't do this but struck lucky with her interviewer).
Anyway, the above may well make no difference whatsoever, but just wanted to mention it because these are a few "If only I'd realised ...." snippets I've gleaned from talking with who were successful and those who were not this year.