Kids that are age 12 can legally babysit another living human being, the Red Cross has a babysitters class for kids age 12.
My thoughts are that if they can legally babysit another person at age 12 they should surely be competent at watching themselves.
Your 8-9 year old boy should be working to having time alone at home. IF you live in a scary area that you won't feel safe then disregard this completely.
You can take the other kids for a walk in the neighborhood and leave the older boy home alone. He has rules of no cooking, no online without supervision including cell phone stuff unless it's where he's locked out of internet and unsafe places without a password, not to answer the door if anyone is knocking even if it's his best friend or the next door neighbor, and he is to do something specific like work on homework, sit at the dining room table coloring, or sitting in a big recliner reading a book.
He should have a list of things he can do so he can have an easy time staying busy. We have trained our girl to make popcorn in the microwave on mini bags and normal bags so she can have a snack plus she can have a pop or juice or water to drink.
You son would be home alone for maybe half an hour. That's a long time for a kiddo that hears strange noises for the first time, the house creaking, the neighbors talking or TV noises if you live in an apartment, it can really play on their emotions if they've never really noticed them before.
He can build up to where you are able to take the 2 younger kids and run to the store, actually driving away from home. Not a few seconds away outside. This is a big step for some kids.
You always have to consider that you might never come back home so you have to teach your child what to do if you don't show up on time or don't call them to check in, you know what I mean. An accident can happen or a sudden illness, anything can happen so kiddo needs to know who's watching out for them if you don't come home when you're supposed to.
I have 4 extremely trustworthy adults that are always on our pick up list. If they go to the kids schools to pick them up there's a darn good reason.
If you don't come home when it's time and it's getting later your son needs a list of several people he can contact and say "Hey, my mom left me home alone and ran to the store. She's not answering her cell and neither is dad, and it's late. Can you come over and get me please?". Then if you come home and he's not there your friend needs to have left a note or something on the fridge letting you know where he is.
This way you know your son is safe and will be looked out for regardless of anything happening to you while you're gone.
He can work up to more and more time but at age 8-9 I'd say an hour is maximum. If he's perfect at it then by summer I'd have worked up to allowing him a couple of hours and maybe more but never with the other kids. He's not old enough to babysit and he's not an authority figure to them yet. They can all still go to child care at this age so they aren't old enough to all stay home every day for weeks on end.