It can be hard for some kids (probably most) because the change in sheer size of buildings, different subjects, new people etc. can make it quite daunting.
Things to settle nerves:
Remind DC everyone is new and keen to make friends (even if they're going with current primary school friends, it's likely that they will want to make new friends also).
Minimise any anxiety about homework by encouraging the use of their homework diary to help keep track of it all.
Get them used to making the journey on their own, even if they plan to walk with a friend it will boost their confidence if they can do it alone.
Get them used to buying things on their own, if they have to use cash to buy lunch or just that simple transaction and interaction which they may not have to do currently.
Some schools seem to initially set loads of homework to get them into the habit of it, others seem to ease them into the routine. Just make sure they're keeping track and doing it, if possible.
Some schools will test with the first few weeks to enable setting, not much you can / should do to prepare for these tests IMHO.
Over the summer before secondary school, we did no academic work. My DCs enjoy reading, so they continued with that as usual. Our main preparation was buying uniform, stationery and a plain tupperware box for packed lunches (replacing "childish" primary school lunch box).
It's a rare opportunity in their school lives to be absolutely pressure-free, so I would let them just enjoy these weeks and do things for fun 