I agree with others, it really is best to have someone to check on them at least once per day. Some things that will help make this easier for everyone:
-Automatic coop door. Means that a chicken-sitter doesn't have to get up a dawn to let the chickens out. I usually ask my chicken-sitter to stop by shortly after dark to make sure that everyone made it in and the door is closed. This won't prevent all mishaps, but I think it's a good balance of chicken care and making things easy for the chicken-sitter.
-Bigger feeders and waterers. If you have someone to check on them daily this isn't really an issue, but I use modified 5 gallon buckets for my feeder and waterer for my flock of 8 that can go for at least a week without being refilled when the weather is cool (the water runs out quicker when it is hot out). Folks with larger flocks will modify trash cans.
-For chicks: are they young enough that they are still in a brooder on heat? If so, I would be hesitant to leave a heat lamp unattended for that long.
I have several friends/neighbors who keep chickens, so they are my preferred chicken-sitters since they will have an easier time troubleshooting any issues. When they aren't available, I ask a non-chicken-keeping friend to check on them every evening and see if one of my chicken-keeping friends is available to be "on call" for the primary chicken-sitter to contact if there are any emergencies.