I would never give a child supplements or even herbal remedies without your Dr approval. Most of that stuff is not even tested on children. Try the V8 line of juices that have veggie juice or there are those natural chips that are made from veggies. My kids think I am giving them potato chips when in turn it is suppose to be 1/2 cup of veggies in each serving.
For your 2 year old have you talked to his Dr about adnoid issues? Does he sleep with his mouth open, congested a lot and snoring of any kind? It could be enlarged adnoids. I totally believe taste buds change. I love almost all foods however was A VERY PICKY EATER when I was a kid. Very picky.
You know I grew up only liking green beans and corn...grew up very healthy, still am and though now I LOVE veggies of all kinds I kind of just have my kids pick at least three favorite veggies and not stress too much about them not eating a ton of veggies. Thankfully they like healthy ones.
With your oldest have you tried ranch dip, ketchup? My daughter LOVES asparagus dipped in ketchup (yes I know it sounds so gross doesn't it?), but if they eat it, who cares what it is dipped in? I make a brocolli rice dish that is loaded with cheese so they don't even realize how much brocolli they are eating.
Hang in there and if kids see you and your hubby eating veggies and you just continue offering them they will come around.
I think too much stress is put on us parents to make sure our children eat these perfect meals, we are their example of eating healthy and all you can do is try.