CHADD just put together a good article about the possible use of CBD oil as a treatment for ADHD - what's know and what''s not. You can read the article here, but the bottom line is that there is not yet evidence that it helps, and there is evidence that it is harmful for children under 16 (that age being the one in a research study...so somewhat arbitrary...the idea being CBD oil and developing brains don't mix). Further - since it's unregulated, you really have to know the product you are buying to know if it is consistent and what you are seeking.
There are other non-medicinal approaches to managing ADHD, such as exercise, mindfulness and, for some, the addition of Omega-3s and elimination of gluten (only for those who have had undiagnosed celiac, which is about 15%of the ADHD population.) These have been shown in published research to be both safe and effective...choose them instead, until more is known about CBD oil.