For 2 weeks before school started I would put up quilts in the bedroom windows to make our daughters room dark.
I suggest you make your room as dark as possible. Make it cooler and put a sound machine in there.
Place the kids on the floor on a mattress, cots or sleeping bags. You need to get them into their own rooms, this is not the time to try to change this, since school will be a big adjustment right now..
In the morning, start waking them up at the time they will need to wake up once school starts. We had to leave for school at 7:30 am, so I used to wake our daughter up beginning at 6:45 am. Then I would give her a few minutes to sleep "a few more minutes"..
I would get breakfast started and make her lunch and then make sure she had at least gone to the potty and brushed her teeth by 7:00 am.
Then she would get dressed.. (the clothing was chosen the night before). Then eat her breakfast.. Our daughter did not like to be spoken to in the morning or asked any questions (she is like me).. So we had an agreement we would not speak unless we were ready for conversations..
By 7:20 we would make sure we had everything for the day. Brush her hair, etc.. In the car at 7:30..
To help your children Wake them up starting tomorrow at 7:00 am.. Give them breakfast and then let them play inside till 9:00.
At 9:00, they need to go outside and play.. Run scream jump, hop, ride trikes, bikes..
At 11:00 get them to put away their toys and stuff from outside. Bring them in get them cleaned up and have lunch by 11:30 .
After lunch, wipe them down (use some good hard wipes with a warm wash cloth)..
Then take them to their sleeping area (room darkened) read them 2 or 3 picture books.. DO NOT ask them questions or get them into conversations.. Have them only listen to the stories.. On the last book, read slower and slower. If they are still awake, tell them they need to stay laying down and no talking.. I used to tell my daughter, mom needs a nap, so please help me, by being quiet so I can sleep.
Then tell them they need to rest if they have not fallen asleep. They must stay on their mattress, sleeping bag.. whatever they sleep on..
In an hour and a half to 2 hours, get them up, give them a snack and let them play outside or inside..
Then take them swimming for an hour or 2. Bring them home, change their clothes, quiet play while you prepare dinner..
Dinner starts at 5:30 or 6:00 Keep the house quiet.. Notice.. NO TV all day..
No horse play during dinner, keep it nice and quiet.. After dinner, get the kids to help clean up..
Then bath times, again no loud talking, no TV, no cell Phone calls, no loud horse play.. Rub them with strong strokes in the bathtub. Dry them with strong strokes.
Make sure the bedroom is prepped.. Dark, cool and white noise machine to block out neighborhood sounds.
Start bed time story no later than 7:30, read books, again do not ask them questions, do not make it animated.. Very matter of fact. Read the last book slower and quieter.
Repeat this each day.. If they cannot go swimming every day.. take them to the park..
The secret is that you have to wake them up at 6:45 to 7:00.. sleeping till 9 or 10 is what has moved their schedule back so far..
For their diet make sure they are eating healthy.. maybe cut out juice so they do not have so much sugar in their systems.. Make sure you look at their diets and they are getting enough protein.. It makes a huge difference.
It can be done. They are just not on a regulated schedule. Children need and thrive with schedules.. I promise..