Little kids dislike food that's good for them

Which sounds obvious, but this research indicates that it's dependent solely on being told that the food is somehow beneficial for them.  The speculation is that kids tend to only track one attribute of a thing at a time. If told it will taste good, they'll tend to believe that. If told that it will benefit them in some other fashion ("this will help you grow strong!"), they'll believe that, too — but at the expense of thinking it can taste good.

So just tell the kids it tastes good, and leave the nutritional advantages to their school Health class later.