C.B.
my neice is this way...her parents are constantly hounding her to eat and she digs her heels in and it is completely a power struggle, which they do NOT win - ever. but they keep bullying, hounding, threatening...it just damages their relationship with her, nothing else. seriously, when she was at my house recently, it was time for lunch and she got that look in her eye, like she was just depressed, but also angry, and seeming to expect a fight...and i just told her, if you're not hungry, you don't have to eat sweetie. she looked at me like i sprouted horns. and eventually she a decent amount.
my advice is to put real effort into having no junk food in the house, offer healthy choices whether it is snack or mealtime. then you don't have to worry. the more you push, the more they will fight it. it is really something you have no control over.