I would stop my dc having a 2nd helping if they hadn't finished their 1st course ie they love the top of lasagna but the bottom gets left. I feel if they're hungry they should finish the whole thing.
Also in our house there is 5 of us and there is never enough left over for any 2nds anyway. They have plenty big enough portions and we have a groaning fruit bowl which they can eat as much as they want.I know this happens in a lot of my friends houses. With the price of food I can't afford vast amounts of decent meat, I make a big pot,pad it out but when it's gone it's gone.
I agree with the op to some extent,letting kids gorge on chips,sweets and crap is setting up problems for later. My dc are allowed everything(except sweeteners and rehydrated fat ) but they know chips, sweets etc are only a once a week thing and they have 50ps worth of sweets not their whole £1 pocket money.
Learning to eat healthily is an important life skill like anything else. Also lasagna weather homemade or not is choc full of calories, fat,very little veg etc. 1 portion has probably got plenty enough fat,salt etc towards a days intake. Kids don't actually need more but going hungry isn't good either so I don't see the problem with unlimited fruit myself.
I love food and want my kids to enjoy good food too( including cakes,deserts,choc etc)but I want them to know how to moderate things later when I'm not around.
If you go down the route of not denying kids what they want it would be chips at every meal,I know one of my sons would love that,so would I if I let myself but I don't. If I did I'd become morbidly obese instead of the healthy size 12/14 that I am. I'm not skinny because I enjoy all foods,eat what I want and don't go overboard on the denial thing but I'm healthy.
I do think it's a thin line though, if you get too obsessive that is seriously not good for kids.I'd imagine it would be easy to go overboard if you weren't careful.