They should be responsible for -
Their own school lunches.
Emptying their schoolbags regularly and disposing of worksheets or work they've brought home.
Keeping track of when they need PE gear, art supplies, etc. for school and getting that ready the night before.
Making their own beds daily.
Stripping their beds once a week, putting on a bedding plus clothing wash, putting wash out to dry, remaking their beds, putting clothing away or in airing cupboard along with bedding.
Keeping their rooms tidy and clean - all hoovering and polishing and window cleaning.
Helping with meal prep twice a week. They should be learning to prepare a whole meal - maybe Saturday lunch to start with.
Loading or unloading dishwasher.
Bringing their own plates, glasses, mugs, to the kitchen after all meals or snacks or drinks, scraping or emptying, and leaving them wherever you prefer to have them left.
You for your part will need to sit them down and make sure they understand your expectations, including performing the chores and doing them without whining. Make a chore chart and a reminder notice about whining.
Take them to task about whining and do not let things slide. Your management skills are essential here, and you need to be on the ball constantly.
Yes, that means work, but it's essential you do this work. You will be teaching your DCs essential life skills, developing their confidence and self esteem enormously, showing them they are accountable to you (supremely important as the teen years approach), and demonstrating management skills to them too.
You will be training them to transition smoothly into the next phase of their lives, where responsibility and accountability must accompany a certain amount of increased trust and freedom.