Isn't that a hoot that their preferences are so specific? My Richard is the same way - squash has to be grated a certain way, turnip/mustard/collard greens have to be a certain size, (and they can't touch each other in the dish!), and he won't eat anything if he drops it or it falls out of his mouth. Jeez.
Anyway, mustard greens are a staple for Richard and Dubs as well. I like the fact that they last so long in the fridge - a big bunch (98 cents at our WalMart) lasts my two boys a couple of weeks. I also feed it to the superworms, and crickets when I have them.