My daughter never had croup but my son gets it all the time. Per my pediatrician's office, the almost sure fire way to nip it in the bud is to expose them to 15 minutes of warm steam, then 15 minutes of cold and then repeat. Every time we've done that except once, the croup was gone. It's an arduous process as our bathrooms are either too large to get real steamy or they are open to the bedroom so it's hard to get a steamy room in our house. We have to drape a blanket over the shower stall and stand in there with him for 15 minutes, which he cries the whole time. I will say, just recently, my husband had the bright idea to plug the vaporizer in in the smallest half bath in the house to steam it up rather than the shower stall thing so that's what we'll do the next time it happens. And when it's not cold enough outside, we have to sit on a chair in front of the half open refrigerator/freezer with him for 15 minutes. He loves that as he plays with all the stuff in there while we're waiting.
If it is cold outside, you do have to wrap them up real good. You just want to expose their lungs to the cold, not their whole bodies, so you also need to dress them in something warm when your standing in front of the open fridge as well.
This is the remedy we've been given by the after hours urgent care line at my doctor's office every time we've called about that horrible barking seal cough. I wish there was a medicine 'cause the hot/cold, hot/cold for an hour can be tiring at 2 a.m., or anytime for that matter. But my husband and I have a system where he takes care of the hot and I take care of the cold and before you know it, our little "seal" is sleeping soundly again. Good luck to you!