How critical is it to test drugs and vaccines in kids before giving them to kids?

According to the FDA:

As of 2008, an estimated 50 to 60 percent of prescription drugs used to treat children have been studied in some part of the pediatric population. Still, the likelihood that a medicine has actually been studied in neonates—children less than a month old—is close to zero.
So nearly a decade into the 21st Century, most medicines intended for children, including many over-the-counter (OTC) products, haven’t been clinically studied in children—and certainly not in all age populations that comprise the branch of medicine known as “pediatrics.”

 In other words, we "guesstimate" pediatric doses most of the time.  Guesstimating an anthrax vaccine dose, particularly since we have tested the vaccine in many small monkeys, would not be a stretch from standard pediatric practice.