If your kids already take a daily vitamin, they do not need additional vitamin stuff in their smoothies.
And if they already do eat a varied healthy diet, they do not need additional "vitamin" supplements in their smoothies.
And, per protein powders, these are typically for adults and the "serving size" is per adults. Not children.
If given too much protein powder, it can overload their kidneys etc. For example.
How much, for example, is appropriate for a 2 and 4 year old child?
You should ask your Pediatrician.
Smoothies made with fruits and veggies, is already "healthy" and full of nutrients.
And per young 2 and 4 year old children, you should go by their ages and what serving sizes is appropriate for them.
We make smoothies with anything we have on hand.
Be it fruits or veggies or Chia seeds or Flax seeds or peanut butter etc.
BUT we do not add.... those sweet things like Jamba Juice does. Like sherberts or anything.
We just use, straight fruits or veggies.