G.B.
I had this problem with my daughter. If there was a light on anywhere in the house she was up. No matter if her door was closed or not. I would get her to bed and start my homework. She would pop in and want attention. It was so hard.
You need to pick a time that is earlier. It could start being midnight. Just get them ready for bed then sit down doing something quiet. Reading a book to them, something that will signal to their bodies that it is time to wind down.
Then turn the lights out and if you need to lay down with them until they start falling asleep. Then do it 30 minutes earlier the next week or month. It takes a long time to change biorhythms
People who work rotating shifts lose part of their life expectancy for changing their sleep cycles so often. It is a big deal to change it and doing it right is very important.
Just keep working on moving their bed time up each month and work hard, no backsliding. After they go to bed you can get up and do quiet stuff. If they start waking up then you are going to have to decide who's more important, them and them getting to bed in the evening or you and your after midnight time.
I assume they are not school aged or they would be falling asleep a lot earlier than this. Once they start school I think a 9pm bedtime is the latest little ones should be up. They can be in thier room winding down with the lights off but they need at least 10 hours of sleep per night. 9pm-7am.
If you guys are sleeping in really really late due to the late nights it's going to be hard on you to change your sleep patterns too. They will have to be up and ready for school so you need to start working towards that goal too.