Becky, I can understand wanting her to be at the table for dinner. Family meal times together are very important to me, I really believe in all sitting together, enjoying one another, catching up, reconnecting, and eating the same meal (as far as possible, I do slight adaptations for DS to suit his personal preferences). I can also see that with two LOs you wouldn't want it to look like one is expected to eat a certain amount and stay at the table whilst the other doesn't need to. I just see it as illness/teething/tiredness and perhaps if you say this when you let her down from the table it will explain the differences in expectations?
I know once DP was snappy with DS for not coming to the table for dinner, a day when DS was clearly poorly, I asked DP if he expected me to sit and eat a family meal and socialise when I am poorly, he said no. Enough said.
if you are worried about the balance of her diet can you maybe give something different at breakfast when she is more willing to eat? Apart from social conventions there is really nothing to say that meat, veg, pasta etc can't be eaten for breakfast or for the snacks so that across the day she gets a balance?