You know your own kids and their attitudes best, of course, but I would be careful about painting the carby foods as being 'special'. My mom did that due to finances because we saved necessary dollars by eating mostly just meals and fewer desserts and treat snacks. So, in my twisted mind, 'special' meant 'preferred'. She also tried to teach us that veggies were healthy and to avoid them would make us grow up sickly. Since I hated all but a few of them, I associated nasty with healthy and sweet with pleasureful eating. The one blessing has been that I've always liked meats. However, I loved sugar enough that even in the late sixties,before all the food hysteria and fads, she was aware that I had an unhealthy obsession. She tried to scare me about diabetes but didn't have enough real info to make it stick. I think she was also trying to teach me that food choices have consequences without making me paranoid or unreasonably fearful/obsessed with food. In the long run, I found that being able to procure the foods I LIKE made me feel in control of my world, so in my young adult years, I gravitated to them at every opportunity; and since that meant having my own car and money, I made my own opportunities.
I guess if your kids are not emotionally lit up by sweets then it's okay. Just watch for signs that they seem to really look forward to the treats from their grandparents and if they do, don't preach at them that veggies are healthy if they don't like them. Instead, sit down with a list of the veggies they do like and don't, and try to add just one, every so often, by finding some way to cook them that will help add them to the good list. For example, if I had known how different cooked and raw broccoli are, I might have been convinced to eat some raw broccoli. It's so much stronger when you cook it and therefore, I still favor it raw.
I don't know what would have controlled my desperate love of sweets but I wish there had been more knowledge out there when I was a kid.