How to get vitamins into your kids?

The challenge though is how to ensure that my kids are getting all the nutrients they need. I have 2 boys 8 & 6 and they both like different foods. One loves meat and fruit and the other likes more veg than meat and no fruit! So neither is getting the right balance. As much as possible we are always working on expanding their food choices, we've tried every incentive going. Star points, pocket money, counting veg.