If I may answer this, re grandparents, my parents are both dead. I would imagine they would have stuffed DS full of sugary delights if they had the chance and I would have been trying to stop them. They certainly did me no favours by stuffing me full of sweets as a child.
However, I of course hugely regret the loss of the sheer amount of love that DS would have had from my mum
My DS does have refined sugar once in a while and certainly I think a small piece of cake at a birthday party is fine. He doesn't have much refined sugar in everyday food as I cook a lot of our food from scratch and buy yoghurts like Rachel's Organic which don't have any extra sugar added at all, just fruit.
Re the need to give careful thought to one's child's diet, there is a current child obesity epidemic, see here.
"England has seen the fastest growth in obesity in Europe and childhood obesity has tripled in the past 20 years. "
and
"obese children could become the first generation to die before their parents."
There is a multi-million pound food industry targeting its wares at children.
Also, if one has a history of food related problems such as obesity and diabetes in the family (and we do), all the more reason to be cautious.
It's a question of getting the balance right isn't it?