Leo, glad you feel great, don't feel bad about it!!!!
Overrun, you must be worn out and worn down my love. Can you think of anything that could be withheld that would upset them? I know its sounds harsh but I'm passionate about children going to bed and staying asleep. It makes everybody feel well.
It sounds like you are doing eveything you can but that they are testing the boundaries. Could you shut the door, put off the light, and keep the door shut, do not respond, walk away. Let them see that no matter how much nonsense they have it isn't working, you will not give them any more attention after bedtime?
And now, a book..........
How about a fresh start? Pick your moment carefully. Start down beside them holding their hands and by saying (in a calm and smiley voice/face) that tonight is different, that we're not going to have nonsense at bedtime aaannnnymore because it makes mummy very unhappy. Tell them that everything is different tonight.
Make bathtime and bedtime different so they see a physical difference not just words. How about blow up some balloons for the bath and buy some crazy soap to make faces and hair on them. Have fun, don't shout no matter what they do. Let them feel really happy and let them see that you adore them having fun. If they are playing really well and laughing you could always say you found a couple of chocolate buttons in your pocket and ask if they'd like to use them up. This is the psychology bit of getting their attention when they are happy and they associate the happiness with you (i.e. they are happy and its mummy/daddy not its telling off time and its mummy/daddy).
When getting dried make it a time they can run about, dance, make noise, sing songs, be silly. This helps run off some energy and marks the contrast of bedtime.
Say bedtime is 7pm, then have them all ready by 6.30pm ish (time it for a particular programme, I think Bear in the Big Blue House is on around then and is quite calm), let them watch some telly (maybe in your arms with the lights very low) or play their favourite, favourite game (a calm one). If you really wanted to make them feel special you could make hot chocolates but this might be a bridge too far .
When the programme finishes, pop the telly off, let the house be calm (I leave music on because I think they like to feel someone is around, a completely quiet house doesn't sound right to kids) and tell them you have had a fanstastic time and than it was lots of fun playing with balloons. Tell them you'd love to do it all again tomorrow night and ask them if there's a funny game they can think of that they'd like to play tomorrow (this lets them see they have the power to have input).
Tell them, its bedtime now and you are really happy with them. Remind them that to do it all again you'd like them to get to sleep quickly so that tomorrow comes faster. Try not to mention it too much though and say your goodnights.
Take a deep breath. Give them a few moments of nonsense before going in. Don't turn on the lights, don't shout, just repeat that you don't like this, what a shame, we've had so much fun and we've had chocolate buttons (a mental reminder of the fun), smile, tell them you wont to be able to do it again tomorrow unless they are quiet and its time to sleep.
Every time after this don't speak, other than to say quietly, its bedtime and return them to bed. Do exactly the same over and over and over. Personally I wouldn't stay in the room because its just another hurdle to get over later.
If they talk to each other from their beds I'd leave them, you are on a winner if you get there!
If they are good, make sure the next night is just as much fun to show them they did it. If they have just as much nonsense, still try to repeat some of the fun the next bedtime so that they associate you as being a fun person and someone they want to please.
I hope I haven't offended you by posting all this. Its just a suggestion in case you haven't tried any of it yet (and you maybe have). As I say I am passionate about children sleeping properly and I know how wearing it is when it goes tits up. Please don't think my bedtimes/bathtime are perfect either, I'm just as close to losing it at this time of night as my thighs are getting to each other .
Stay strong.