Well, you could always do what we did and have a few more...and then they can poop each other out!
I know what you mean about looking at other kids and marvelling at their ability to sit nicely and quietly...my nieces are the same ages as my older two...and I remember watching them in wonderment one day, because the two of them sat still as mice FOR AN HOUR, colouring. No wonder my sister's house is cleaner than mine! But actually I wouldn't trade my rowdy rambunctious little rapscallions for anything in the world, mess and noise and general level of energetic chaos notwithstanding, and I'm sure you feel exactly the same.
Try to get some rest when you can, and get lots of protein!
peace
minnie
