Reducing the amount of processed foods in your home will have a huge impact, and I commend you for making the effort. Trans fats are metabolic poison and the only truly safe level for human consumption is zero. You may not be able to get every single gram of trans fat out of your children’s diet, but it certainly doesn’t hurt to minimize it as much as possible. And high fructose corn syrup (HFCS), while arguably not that much worse than regular table sugar, is being consumed in amounts that are simply dangerous for our health and well-being. The fructose part of both table sugar and HFCS can elevate triglycerides and also lead to insulin resistance.
That said, your wife’s point is not entirely without merit. If you demonize foods that kids love, it’s entirely possible that they will rebel and consume even more of them. The same thing used to be said about cigarettes and alcohol when I was a kid, leading some well-intentioned parents to let their children try both substances in an attempt to demystify them. Did it work? Who knows?
Here’s what I advise: Find a middle ground. Don’t keep foods that are loaded with sugar, HFCS or hydrogenated oils (trans fats) in the house. Sure, the kids will complain at first, but most will eat whatever is there, giving you a great opportunity to build in some good lifelong habits. (When a child comes home from school and opens the refrigerator and all she finds is a bunch of delicious-looking fresh grapes, that’s what she’ll grab, believe me.) Don’t forbid the kid-foods; just explain that they really aren’t that good for you, so they’re limited to special occasions and treats. Try to find tasty substitutes for the worst offenders (baked “fries” instead of McDonald’s, Larabars instead of candy bars) and don’t make too big a deal out of it. Children model what they see around them, so if you and your wife are eating the right things, eventually the kids will, too.
As for your eight year old, I wouldn’t worry too much. Make the new foods you want her to try seem like fun and eventually she’ll try things in her own time. It may seem like all she eats right now is ketchup and pizza, but trust me, when she’s on her first date 10 years from now, she won’t be ordering McNuggets.