As soon as my wife realised it wasn't just simple vomitting she called a doctor and they told us to go to hospital, so we were there within 3 hours of the first episode. My wife stayed with them in hospital, but because of corona I wasn't allowed to see them - which wasn't particularly nice for me and I'm sure was very traumatic for our kid.
We have all recovered now, but yes it's somewhat surprising how quickly young kids can go downhill.