C.,
There are some great ideas on here already, I just wanted to second some of them...
First Darken the room, there are some great darkening products from blinds to curtains, or if on a budget you could use card board or foil. I just saw in the one step ahead mag (online too) 2 blackout drapery liners for 40.00 you can cut them to whatever length you need. 60"L x 27" W
(I also liked the idea of the night light that changes colors to let them know when its okay to get up.)
Second, get a clock with a face. I put masking tape (cut thin and colored in) to mark the time they are allowed to get out of bed. Of course bathroom is fine, but if its not past wake time, they have to get back in bed and close their eyes.
Third as soon as I hear them up, if its not time, I take them by the hand and tuck them back in bed, whispering that its not time to get up yet. This may sound funny, but I try to look sleepy still, so they know that even mommy is not ready to get up.
Fourth, get a white noise machine or a fan to put in their room. It will drowned out any birds or cars or laundry starting etc...
This has worked very well for my three girls and I am sure it will for the fourth too when he is older. They sleep from 8:00 until 8:00 or some times 9:00 if I let them!
my first and second sound just like yours, the second was a very loud child and didn't get the concept of not waking the other up for a while. But we practiced a lot on trying to be quite and whispering and holding still, even when it wasn't time to sleep. This helped quite a bit.
Good luck, may sleep be with you!
E.