E.B.
This actually happened to me when my son was one and my daughter was 4. It was REALLY weird. They were fine all day and then threw up in their beds at night. My husband and I then came down with an actual stomach bug, so that is definately what it was. I started them on the BRATS diet and it seemed to go away on it's only shortly after. You can look up suggestions to go along with the BRATS diet online. Good luck!