Good idea, here are my tips!
I'm thinking practical stuff (like what do they need but no one tells you in advance , what schools say they need but don't) planning travel to school etc.
What they actually need will vary from school to school, if you can get advice from someone already there take it, but otherwise start off with everything to avoid panic and nagging doubts about whether they will get a detention on their first day for not having science goggles for example. (They won't, but the last thing you want is for them to be worried that they might.) Also, go to the second hand uniform sale and buy, buy, buy, you might not think it necessary, but it will be!
Make sure DC know the journey and know a selection of what if alternatives. Maybe even print a little sheet for them to fold up and hide somewhere (humiliating otherwise) so that if someone jumps under a train on day 1 (DD1) or is a there is a 'fire on the track' (DD2) on day one they know what to do.
Emotional stuff, getting them mentally prepared for what is to to come. Making new friends, keeping/ditching old ones.
DD1 went with 2 friends from primary, DD2 with none, life was much easier for DD2, easy advice 'try to make friends'. Encourage DC to be open, pleasant and kind and watch, hope and step in if necessary (but only if you think there is a very strong chance of a positive outcome).
Educational stuff, keeping the brain ticking over during the summer by doing a bit of work or letting them have a well earned rest. Looking at subjects they haven't covered before or leaving it until they get there.
Let them rest and have fun! DD1 was convinced she was a numpty when she started doing French and Latin courses at high school which the prep kids had already done. However, in the end of first term exams she scored higher than almost all of them. Kids catch up, teachers teach, the over prepped may struggle. Make sure they realise that kids will have been taught different things so far and discourage them from making early judgements about their own capability (let alone anyone else's).
Good luck, it is a tiring time, but making sure DC feel supported is key. Finally, try (I failed) not to interrogate them about every detail and then get in a panic. They will be tired and will need to come home to a respite from school environment, particularly if they feel they are floundering. Kids pick up on stress so try to be chilled. Easily said and not what I did first time round but hey ho, hindsight and all that!