Good post granny activist, as it reminds us not all our generation had the best diet in the world, but here we are, enjoying life.
I'm with the group who suggest you to avoid unnecessary conflict with your granddaughter/s parents pynkim. Whilst the diet doesn't sound ideal, it isn't dreadful, there is a routine, your little granddaughter is loved and well cared for. Meal times can be stressful for children and parents, and creating a relaxed and positive atmosphere around food is I feel the most important thing for your granddaughter right now.
My views are influenced by childhood experiences, as well as being a mother and grandmother now. Throughout my childhood, my paternal gran visited every Saturday, money was tight, and her bags brimming with goodies were always eagerly awaited by we 3 children. Her preference was to buy us treats, biscuits, chocolate, sweets. I'm sure this was because her own childhood hadn't included such extravagance. Mum was a fanatic on the healthy eating/strong teeth front and made sure we had lots of vegetables and fruit, whilst biscuits were eaten when gran brought them, then no more till the following weekend. Gradually grannie began to include oranges, apples, pears, banana's, even pomegranates, in the bag of goodies. My mum and her mother in law were very different people, and 60 years on, the way in which they managed what could have been a conflicted relationship is still reflected on with humour and affection by the remaining family members.