Thank you for your question!
There are no products, vitamins or supplements that have been shown to be effective or beneficial for the immune system in adults and children.
Instead, we should look at non-pharmacological measures: make sure your children eat plenty of fruits and vegetables, get adequate sleep, and ensure good stress management if possible, among other things. If one of the children or members of the household is ill, simply maintain basic hygiene measures: avoid hugging if possible, do not share towels, and wash them often.
Finally, if your children are prone to colds, I strongly suggest, as a maintenance and prevention measure, sinus hygiene with a saline solution like Hydrasense or Sinus Rinse. This is the best option to prevent the accumulation of secretions. There is no maximum during the day, but it should be done at least 30 minutes before bedtime.
I hope I've been helpful, and I'm available if there's anything I can do
Tom Samaha, pharmacist