I would strongly suggest you not feed charcoal to a baby, as one of the side effects is that it could cause diarrhea or vomiting.
Also, babies are much more sensitive, and in their small bellies, could cause a bowel obstruction, and you don't want that.
A bowl of pureed pumpkin is much gentler, and would help a lot. We use it in our clinics for dogs with diarrhea, and it's really good for your baby anyway. You could flavor it to your taste as well.