Adding to what Shannon said, MAKE the older children read for a half-hour before bed, and read to the younger ones that long as well. It gets them to calm down their bodies and do something educational.
This might take a few weeks (assuming you have them on the weekends), but each evening try to institute a time when you discuss what rewards they will get for cooperating with pre-bedtime and bedtime routines (like getting to go to a toy store and have 10 bucks to spend, getting to go to the movies). Better yet, make some sort of chart/visual where you can easily remove a name/reward as necessary, and let them write/draw it. As soon as the rewards are established, put your visual in a convenient place, and as soon as they don't cooperate, escort them to the visual and remove their part so such a manner that they know they've messed up. They will cry and have a fit, it will be a pain for your head and ears, you and your husband might have to be separate for a few weekends (rotating between taking the cooperative children out and keeping those who did not cooperate in AND not rewarded) but do not reward them for behavior you do not want. And make sure the removal of the reward is truly a punishment; what will affect one child will not affect another (for example, sending me to my room and not letting me go play with friends was a fate worse than death for me, but my sister preferred to play in her room - though removing the toys might have helped).