i hear you hon
used to have the same probs when my twins were the same age!
in the end if they didnt eat their dinner, i would put it in the fridge and if they got hungry later, then they could finish their dinner then. They other thing is to try offering them dinner at a different time. In the end we use to offer dinner at say 5pm instead and then supper if they ate all their dinner , just before bedtime.
so it would be lunch at 12pm
arvo tea around 2ish
dinner at 5pm and supper at 7

Hope this helps and GL!
when they start kinder/school you will find that this issue settles down more as their lives become more regimented!
hang in there!!