I have 4 children, ages 3 to 10, and I can relate to your concern. I BF all of mine, and transitioning to other foods was sometimes a challenge. I am now a nutritional consultant working with a dr. of nutrition (we do healthy cooking classes in DeKalb & metro area); therefore, my view of what to feed children has now changed somewhat. My biggest understanding now from a health perspective is that every food is not for every body...particularly not cow's milk. You didn't ask to get this health-conscious, but I thought that I might share this information anyhow. I have now learned that eating according to your blood type is very important because certain foods are either beneficial or detrimental for your particular make-up. That's why so many children and adults have allergies and health issues, and may not be able to figure out why. For example, blood type O's & A's are not to eat dairy products.
You are right on point to make getting "nutrition" into your child the top priority and point of eating. One thing that your child might take to is a home-made smoothie. These are tasty and a great tool to 'sneak' some nutrition into them. You can use frozen or fresh strawberries, bananas, etc. as a base. But the next point is to add some 'nutrition', whole foods. In place of grinding up a vitamin you can start with a whole food such as a high quality flaxseed, wheat germ (both of which I can recommend the brand). You can also add a high quality supplement for children that has all of their vitamins & minerals, etc, but in a more natural, healthier formula than even baby formula. (I can recommend that supplement as well.) Add filtered water & ice (opt.) & blend 2 min. Also, get a food processor and make little meals out of fruits & veggies i.e. papaya, pinneapple, broccoli, etc.
You can contact me for more info if you like: ____@____.com, ###-###-####.
But whatever route you take, understand that you are ate an important crossroads in formulating your child's future eating habits and health. It is easier to start a good habit now than to break a bad one later...I know from experience.