I gave my 4 month old culturelle probiotics for adult (about half the dose) only to later see its not for use in kids under age 1. dangerous???

No worries, but...: There are many Lactobacillus types, and various studies have tried amounts of 1-10 billion CFUs (colony forming units) in babies, without seeing any bad side effects. That being said, there is seldom much research that one can find on one particular brand or strain of Lactobacillus use in babies... so no worries, but "user beware".