The bus thing can be a real worry for the kids. Something that can be a real help is to find out where the terminus of your dc's bus is. So if the last stop is Newbieville, it helps them to know that, because the bus will probably have that on the front.
Agree in advance what will happen if they miss the bus, it will help to put their mind, and yours, to rest.
It is probably easier for them to have school meals at first, unless there are diatary reasons not to do so.
Mark everything with their names. especialy the school shoes.
Find out the name of their form tutor/ Head of pastoral team asap.
Get them to do their homework on the day they get it, if at all possible.....so much easier for them, and getting them into good habits early will help later on.
Tell them not to panic. All those sixth formers were once as small as they are.
If they get lost, find someone with a badge (most staff wear name badges), teacher or prefect if the school have them. Eveyone gets lost some time.
Tell them to have fun. They will be fine.