The Recreational UAS Safety Test (TRUST)
To legally fly a drone recreationally in the USA, whether you are a child or adult, you are required by the FAA to obtain a TRUST certificate. This is free and easy to get, and gives you the basic knowledge you need to safely operate your drone for fun. The FAA has partnered with a number of approved providers to offer the course and test online free of charge. I recommend this course by the Pilot Institute.
Applicable to the USA only. Not suitable for other regions.