Consider getting the kids an alarm clock! Stay with me here and you'll see what I mean: Your 2 oldest should be old enough to pick up on this, and this is what you do. Show the kids the alarm clock in your room and explain that this is what big kids and grown ups use so they know when it's time to wake up. When the alarm goes off, it's time to get up. Do you want an alarm clock just like mommy and daddy? This means you'll be a really big girl/boy!!
Take them to the store and have them pick out a simple but neat alarm clock that they like. Go home and practice with it. Set the alarm (music preferably) to go off in a short time and tell the kids to pretend like they're sleeping in their beds, when the alarm goes off, they can get up. Then, do this for all sleeping times (night and naps), and you set the alarm for when they should get up. Now, you need to explain that they stay in their bed until the alarm goes off, even if they wake up first.
Now, you will know when the kids are waking up each day, and getting out of their beds. And even if they aren't sleeping the whole time, they will at least stay in their beds/room until the alarm goes off. If you want, give an option to the oldest to read books quietly in bed until the alarm goes off if they wake up first. It won't be long and you'll have your quiet time again, and they'll be much more independent!! Yeah - this is good for you and you're teaching them how to be resposible for themselves at the same time. Good luck!