Vitamin C for Infant and Toddlers

First I would advise you to talk to your children's doctor about giving them that. Plus do you sanitize everything? If so please try not to do it as much. The things you are killing help your little ones build a immune system. My daughter has been sick once and she is 13 months. I don't sanitize things around the house more than once a week. I do a good cleaning on Sundays. She is out and about with my husband or I everyday so she has the opportunity to pick things up but she has be given the opportunity to build up her immune system.
Plus they have all the other kids if they are in preschool to battle with. Good luck!

my 4 year old takes one chewable vitamin c everyday along with her normal vitamins. for your 5 month old use poly vi sol, for the 20 month old you can use the same as the baby or a chewable one.

I used this brand for my son when he was a infant: http://www.country-life.com/category.cfm?Category=16
Now he won't take it (he's the same age as your middle).
I give him gummy C's by Rainbow Light.
I know the gummies aren't recommended for that age but I always watch him while he eats them.
I also give them D.
HTH!
Oh and Elderberry syrup is another thing they can take to combat the germs! :)

Remember that Vit C is a water soluble vitamin, so excess will be peed out... Best defense against cold germs is handwashing (and face washing for little ones). But I know about germs that you can't prevent. My kids had a lot of day care colds too. Oranges (yep the real food) for the older ones and breast milk (or check baby's formula) for Vit C level.