I would look at the labels of everything that has been recommended here. If there is any sort of warning label (e.g. Flintstones definitely has one), then I wouldn't give it at all. Many pediatricians will tell you that these products are not properly absorbed so you are just creating "expensive urine" and wasting your money. Kids are not getting their nutrients from food (neither are we!) because the food is either overprocessed or it is fruit grown in depleted soil or vegetables picked early and shipped 1000 miles so there isn't a vitamin left in it. That's why we see so many issues related to insufficient nutrition.
I would steer away from any pills or even chewables - the absorption rate is very low. And if stuff tastes yucky, what are you accomplishing except creating a battleground that is frustrating for you and for the kids. It teaches them that what's good for them tastes awful and what's bad for them is delicious!
All my friends have their children on Now for Kids from Reliv - it's a unique blend of everything they need in a highly absorbable formula. I just came from a conference in which various MDs presented their findings on how this product saves the lives of so many kids, makes antibiotics more absorbable (especially in poverty-stricken areas where there is more disease), and they showed us before-and-after pictures. It's the best thing you can buy, and it's a delicious vanilla or chocolate flavor that you mix in milk, water or whatever. I can give you more info and connect you to other parents whose kids have had a 180-degree turnaround. Love to share.