Elderberries are kind of fickle 🥴 My gramma used to make syrup out of them but a lot of the times itd just have to wait to be thrown out. Kids dont like eating "healthy" stuff on its own. You can try adding foods high in vitamins/nutrients in what youre cooking (discreetly still). I stocked up on superfoods like flaxseed, quinoa, hempseeds and goji berries from https://www.vitaminexpress.org/uk/. I add them whenever I can to noodles/porridge. Its made it so easy for me to feed my kids simple but healthy dishes. Plus, it leaves space for a lot of customization bec you can tweak the flavors to your liking.