This sucks for the child, that's for sure. I worked as a teacher and was so disgusted by well-meaning parents who sent sweet yogurt drinks with cookies as lunch. For the science fair we made a display of various "healthy-seeming" snacks/drinks with a ziploc bag of the equivalent amount of sugar under each item to really drive the point home. I hope I made a difference.
It's one thing to give your child this stuff as an occasional treat, or dessert, or indulgence. But to do it thinking it's healthy is just sad for everyone.
I have to think that your relatives just don't know how bad soda really is. I know it's hard to believe in this day and age, but like Retiredat63 pointed out, first eggs were bad, now they're good. I think people get overwhelmed with all the information and just kinda block it out and look for information that supports their bias.
So as to the question whether or not it's child abuse, it's a really tough question. Is pushing your child to be a great piano player abuse because of the stress you're adding to their life? We all know stress takes decades off of people's lives. What about pushing them to achieve high grades? I think we have to look at it from the perspective that everyone's doing their best and find ways to educate and lend a helping hand.
Who knows why people beat their children, which we've all agreed is child abuse. I have to think they are all victims, really. I mean, no one does that and feels good about it. Maybe like .01% do. How to solve THAT scary problem? I don't know.
I agree with the above poster with where to draw the line. If they're on a medical protocol and directly going against that, let's call it abuse/neglect. Otherwise, I think we have to respect our individual freedoms. Otherwise what's the point? Let's just ship them all off to government-run boarding schools after they're born.
I mean, individuals who are close to the family should do something, but not the state. They should set an example. Talk about what they've learned about how damaging sugar really is.
I feel like these are a lot of words and I didn't say much. Tl;dr: it's a tough situation.