Yup, several of these kids sound like mine! DS7 is skinny--he hasn't gained weight in two years, just a pound or two up and back down again--and can't focus on eating. He eats a Toaster Strudel for breakfast, and I sit next to him saying "eat...eat...eat" until he's done. I don't know if he eats much lunch or not--and when I pack his lunch because they're having something he won't eat, his lunchbox comes home completely empty so I don't know what he ate there either. I know in preschool it was quite a struggle for them to get anything down him at lunch, and that was at a table with the teacher prompting everybody to eat.
Funny thing, a favorite story in my family was about when my uncle and his family came to visit when I was about 6, and my little cousin was about 4. My mom would be talking away to someone, pause and say "eat, Sam" to me, because I wasn't, and pretty soon my little cousin picked it up and started saying "eat, Sam" as she shoveled her food in and out-ate everybody. I guess DS7 comes by it naturally.