We have lots of food allergies here. Mine come out on my skin. John's in migraines, although we're not sure of the culprits with him. For our daughter, her allergies make her vomit...for hours. She can't keep anything down and it's terribly worrisome, but each time, she's in bad shape overnight and the following day seems to be okay. By the following evening, she's fine. One reason why is this nifty tea that both hydrates and replaces electrolytes. I call it homemade gatorade since it does what gatorade or Pedialyte professes to do without the chemicals or the colorings.
1 part filtered water to 1 part chamomile infusion
Unprocessed, uniodized sea salt to taste
The chamomile reduces inflammation and irritation in the stomach and gut. The salt is the critical part. If your salt is clear or white and the crystals slide easily over each other, it's processed. Avoid that stuff. The salt you want is grey and cakey and would be called "Celtic" or "unprocessed" sea salt. My favorite is called Esprit de Sel and is in large rocky chunks, but there are many kinds. The good stuff has a mineral composition not unlike human blood, which is why it's so good to ingest when you've been losing fluids. It even tastes better than processed salt, probably because it's not just sodium, but magnesium, manganese, zinc, etc.
If a baby or child has been throwing up and can't keep anything down wait until they can take a gulp or two of liquid and feed this to them by the single spoonful every five minutes. When it looks like they can keep it down, let them sip it slowly until they're feeling better. Serve it to them for the rest of the day alternating with plain water.
It's also great after heavy exercise.