Oh wow thank goodness you only gave them a little taste. I bet that's what caused the upset tummies and not the Frontline. At least now you know so next month when you use the Frontline and if they don't vomit you are good to go. If they vomit then you know for sure it's the Frontline and then you can contact your vet. We use Revolution for puppies and never a problem with little Izz.
Glad that they are all OK now.