Q: How can I keep my baby hydrated when he's vomiting and can't hold any liquids down?
A: The key to hydration at any age is to keep the quantities of liquid very small. Don't allow your child to take sips (or gulps) of water or an electrolyte drink; rather, give only a teaspoon every five to 10 minutes or so. When he is able to hold this much down, increase his intake to two teaspoons. Yes, he will be thirsty and unhappy, but if you allow him to take sips, he'll get tablespoons of the fluid and throw up again.
As long as your child is holding down teaspoons of fluids and peeing and smiling a little, you probably don't need to call the doctor. But if he still can't keep anything down after a few hours, he hasn't peed or his lethargy is worrisome, call your doc.