I don't force DD to eat or insist she eats the whole lot but I don't offer an alternative. I know what she likes & dislikes so I wouldn't give her anything that I didn't think she'd eat. If she refuses dinner, that's it. Nothing else. I've been doing this since around 3 I think, can't remember exactly, just when I knew she understood.
I would of course never insist she ate it all, my dad's parents did that and my dad still feels that he needs to eat everything in front of him. It's a terribly unhealthy habit.