What I did with my kids was to put Lugols in their smootie, juce, milk, tea etc. Started with one drop, then two.
Ascorbic acid or magnesium ascorbate in fruity drinks work well. Natrium ascorbate in savory meals.
Vit B, liquid versions available, NOW food has a good one that taste good (stevia as sweetener)
Selenium from a capsule sprinkled on food.
Magnesium glycinate, malate, citrate, also easy to fortify their det with.
A little boron as well, and you are there.
Start slow, they have a delicate palate, as soon as I overdid anything they refused.
Ps now food also have a zinc lozenge that taste good, if infections are a problem, also has vit c in it.
Andy82 had some good posts of how he supplemented his kids.