I hope you don't mind me popping in. I was in your position this time last year waiting anxiously for GCSE results and I was relatively clueless about year 12.
If any of your DCs are moving to sixth form colleges, sort out a decent passport photo which they can live with over the next few days. They will need this for enrolment, rail cards etc. They won't get their GCSE certificates straight away, so take a couple of copies of their results slips for college admissions. They will probably need photo id or a passport on enrolment day.
The government has just introduced a 16-17 railcard which can be used for buying cheap season tickets until their 18th birthday. This is more cost effective than a standard 16-25 railcard as it's designed to give a discount at peak times, not just off peak. Some colleges have negotiated scholar season tickets which are also good value for term time travel.
Get your DC to sign up for Unidays. This will give them 5% - 15% discounts with lots of retailers and great discounts for eating out. You can usually take advantage of Unidays codes they give you for your own online shopping too.
If any of them are interested in applying for medicine, vet school or Oxbridge, they will need to start building up relevant experience from the start of year 12. This all needs to be subject related, as these degree courses aren't interested in D of E, head boy/ head girl, sporting achievements etc. but super curricular activities like relevant work experience for free Cambridge subject master classes in the spring term.
Don't worry if they only want to take 3 A levels or quickly drop a 4th subject. Very few people take 4 now with the exception of further maths. They can often chop & change for the first term or half term,. Lots of people start one course and switch to something different.
Good luck on Thursday. We were leaping into the unknown last year, but it was nowhere near as bad as we thought it would be in the end.