At age 8 I was doing laundry, cleaning the bathroom, and helping with the dishes. Age 9 I had to bathe my younger brothers. Age 10 I made my lunch and my younger brothers' lunches for summer day camp every day. At 12 and 13, it was my job to clean the kitchen by myself every night. We had no dishwasher, and I was grounded one day for every dirty dish found in the cupboards. My mother, of course, has no memory of this.
Your children and your husband's children are plenty old enough to have more responsibility. And don't pay them for doing these things because nobody gets paid to do house work. If you want to get paid to wash dishes, get a job in a restaurant.
To ensure the work gets done, you have one weapon in your arsenal that your parents did not: the WI-FI password.

At her household, SGD-12 is expected to cook a simple dinner once/week, dishes every evening, laundry, clean bathrooms, vacuum, clean her room. At SO's house she is expected to ... uhm, flush the toilet ... and clean up any messes she makes if it is discovered before she leaves ? (Granted he only sees her for one overnight/week, but it's an issue.)

My BS-14 shovels snow (our house and my mother's), shovels horse poop, cleans his bathroom, walks dog; makes his own snacks including macaroni/cheese, ramen noodles, sandwiches etc. Special dinners he makes the pies --- the last one he made was a lime cheesecake pie, yummm. Less consistently, BS will do the dinner dishes, his own laundry, cleans his room and helps my mother for a few hours/week.

When I was young (elementary school), I set the table daily, did the dishes every night, took care of the animals and cleaned my room. Saturday morning I washed the kitchen floor, dusted, vacuumed main living areas and cleaned the main bathroom.

At their ages, they can do just about everything regarding household chores. However, since your DH isn't on board, it won't work. You'll be the only one fighting to give them responsibility while your DH sits back and lets you be the bad guy.

At 12 I was doing everything except cook meals because there usually wasn't enough food for that so we'd always go out for dinner.

I have no advise, as everyone has already said "hubby is not on the same page." You are wasting your time.

They need to be taught how to do each chore.
They need to be taught how to organize themselves to do each chore.
They need to be taught how to enjoy doing some chores - might as well have fun, right?
They need to be taught how to arrange their days/weeks so they can manage the responsibility of chores, school, social life, down-time.

For that to happen, they either have to be self-motivated, or parent-motivated.

At those ages, there's no reason they can't do everything an adult can do, chore-wise.

Not on same page? I'm curious as to DH's reasoning behind his objections. The few tasks you mention implementing sound very reasonable.

Like others have said, if your husband isn't on the same page you're probably wasting your time.

I've tried to implement chores for SD7, and my husband is on board when we discuss it. But when it comes time to actually get her to do anything, he thinks the things I've come up with are too hard for someone who is "just a little kid", as he says.

There was one day that he had her do some dusting in the living room for maybe 10 minutes. Later on that day I reminded him that she needed to clean up her mess in the toy room, and he said, "But she's been cleaning all day!"

I grew up on a farm/ranch so I was used to doing all kinds of chores by about age 5. My siblings and I had at least 1 hour of outside chores to do on school days (feeding the animals, shoveling manure, etc.) and bigger jobs to do on weekends that took several hours each day, if not all day (moving cattle from one field to another on horseback, branding cattle, doing vaccinations, setting up corrals, shearing sheep, fixing fences, etc.). I can't even take my husband seriously when he says that my suggestions for chores for SD to do are too hard.

It's definitely not too much to expect teenagers to do the chores that others have suggested on here.

Agree with others.
We had "chores" for a while, begun by me, horrified that SDs didn't do/help with ANYTHING.
They worked!
Until the SDs started complaining (and they got money for the chores!) that they thought the chores "took too much of their time." WTF? Clean room once a week was "too much time" out of their hectic day.

Then DP backed away from it and all of it stopped.

At ages 10 and 12 they have NO chores still.
The only thing they are expected to do is clear THEIR OWN place at the table after dinner.

This is a major reason why I disengaged. I do not clean one iota of anything up after them. If I have to (in shared living space) I either trash it or donate it.

If your SO isn't 100% on board forget about it.

At that age they should be helping with cleaning of common areas of the home (bathroom/kitchen/living room) maybe not the ENTIRE room but at least sweep/mop/vacuum the floors, dusting in the living room, wiping down counters in the kitchen and bath.

Also helping outdoors, raking/mowing the yard, shoveling snow, etc.