I wouldn't put different foods on separate plates, just put them separately on the same plate. So if they don't like bolognese sauce, put the pasta on one side and the sauce on the other rather than mixing it all together. Doesn't matter if some bits are touching. That way they can eat the plain pasta and may eventually end up eating bits of plain pasta that have sauce on them etc. It's about desensitisation, I think.
Agree with the poster who insisted on them having bits of everything on their plate -- I tell mine they don't have to eat it, but it's not going to jump off the plate and bite them, so there's no need to get hysterical about it.
You can't offer extra food before bed, or let them help themselves to food from the fridge/cupboard, otherwise they'll just hold out for that. Be very matter of fact and say, "Of course you're hungry, you didn't eat your supper, that was a bit silly, wasn't it? Never mind, tomorrow you'll remember to eat a bit more and then you won't be hungry". You may just have to tough out the early waking for a while -- 6am isn't that early in the great scheme of things.
It works much better if you all sit down to eat together, even if you only have a small portion and eat your main meal later. You could even sell it to them as 'grown up supper'. Cook whatever you're going to cook (bolognese, let's say), put the pasta on the plate, a small dollop of sauce next to it, and serve raw carrot/pepper/cucumber as the vegetable (also on the same plate). If they kick off, tell them just to eat the parts they like and leave the rest. If they really kick off, take the whole plate away and tell them supper is over cos they're obviously not hungry, and it's now bathtime. They'll either back off superquick and ask for the plate back or they'll throw a major screeching tantrum. It is one of those things you only need to do once or twice before they get the idea (but you do need to really mean it).