My 3 year old son has been vomiting for the past 4 hours and can't keep down any food or fluid. at what point do i need to worry about dehydration?

12-18 hours: An otherwise healthy 3 year old can probably go 12-18 hours without much oral intake. It also depends on how much other fluid losses he is having - is there alot of associated diarrhea?Let his stomach rest with no input 2-4 hours, then start a tsp at a time of pedialyte or have him lick slowly at a popsicle. Do this every couple of minutes to get tiny amounts of fluid in frequently.