Sausage roll aside... this is what I do with my boys...
i offer them choices for their meals - would they like, weetbix, toast or porridge for breakfast? whatever they choose, they need to eat. if they don't finish that meal or what I consider a good amount of it, then they don't get their snack, they have to wait till the next meal. it's taken some getting used to for them as they were used to just getting snacks whenever they asked for some but I found they ended up just snacking all day and not eating meals. So now they have set meal times AND set snack times. If they want something outside those times, it depends on what they have eaten that day - also on behaviour sometimes they get a treat.
Food wastage has decreased heaps. i still chuck out heaps - the chooks get it now though - but no where near what it was before I got strict with them.
also, portion sizes... I was once told that a good guide for a portion size for a child is the size of their palm - remember they are much small than adults and don't need anywhere near as much as we do and even then most adults eat more than they need anyway.
ETA - they don't get a choice at dinner time. they eat what mummy makes! I'm not going to be making 4 different meals!