You don't need to unless there's proven deficiencies, as in you've had bloods taken to confirm. They should get everything they need from their diet and dosages for some supplements need to be given cautiously in toddlers
You don't need to give vitamins unless they're deficient. We never have.
We give vitamin D during winter, it’s a liquid and I just add it to his water with breakfast. In the past, I’ve given pentavite as he was an extremely fussy eater. I’d consult your doctor first as a balanced diet should come first.