If you haven't been there before, enter the school postcode on google, click maps and print the streets around. You need to know how to walk from where you park.
If you are a second sitting (not a 9:00 - 9:30 start time) , expect to find parking harder, leave more time so not rushing
When you park, have calm time for a few minutes, eat something (just a little snack, a mini-sausage roll, or fun size chocolate bar etc), also sip from a drink. Treat as settling nerves time
Before you leave make sure you have everything you need, eg 2 pens, 2 pencils, calculator (if required), entry form / candidate number
Try not to mention anything about how important it is whilst queuing (don't want to increase stress levels), remember everyone else in the queue will be nervous (but pointing that out is a mistake)
Don't look worried or be tense (it will be spotted by your child). Think of something else if you are worried such as where shall I stroll to, find a corner shop to buy the reward bar of chocolate for when they come out, basically think of anything but their future and being in an important queue if it stresses you. If you really struggle to cope, then try and get a spouse to do the drop off
Dress DC in comfortable clothes, its not a fashion event, ideally something with some buttons, and thin cardigan that can be removed, exam room temperatures vary a bit and not feeling too hot or too cold is important.
Tell them to ask for the toilet when they get in before the exam (don't want the I think I need to go half an hour in) and can be a rush for toilets between exam papers
Remember they can only do their best, however much you want to find out afterwards let them answer without pushing, don't be alarmed if you hear the question about X was difficult, its natural to remember the ones you couldn't do first as they might have spent some time at the end going back to the questions they didn't fully complete.