My nephews (7,9,11) lived with me for a while and it took a while to break their poor dietary habits. We had no soda in the house, only water and milk and OJ. I kept water in an old jug in the fridge as well. I also fostered boys between 1 year and 6 years of age and ALL were hooked on soda, too.
I would not coerce them to drink anything - but would put milk and water on the table for them. They can choose to drink either one or not drink anything at all. Most said they wanted soda, and I'd say, we only have water and milk. Pick one. It took up to three days before they just accepted it.
The other battle was over junk food, starches, etc. Macaroni and Cheese and Pizza are not meals for growing boys. But that's what they wanted and in many cases, that's what they had the taste for. I'd have to try many different meats prepared different ways to find the ones they liked, then work others into the meals with just small amounts of mashed potatoes and a green vegetable like broccoli or squash. This would take 2-3 weeks.
Usually breakfast with eggs and bacon/sausage would be when they would catch on, then a healthy sandwich lunch, then (Finally) they'd take to dinner. Then, I'd get them to eat meat first, then work on the other items.
After two months of meals high in protein and with little sugar, their bodies would start to fill out and grow.
