Cannabidiol as add-on Therapy for Children With Refractory Epilepsy (CBD-uN1que), a High-quality Individualized Approach: a Series of N-of-1 Trials

The goal of this clinical trial is to learn if cannabidiol ("CBD-oil") works to treat severe epilepsy in children. It will also learn about the safety of cannabidiol. The main questions it aims to answer are: * Does cannabidiol lower the number of seizure in children with severe epilepsy? * Does cannabidiol effect other outcomes such as behaviour, sleep, communication, activities? * What medical problems do participants have when taking cannabidiol? Researchers will compare cannabidiol to a placebo (a look-alike oil that contains no cannabidiol) to see if drug cannabidiol works to treat severe epilepsy. All participants will receive both the cannabidiol and the placebo during different periods. At the end of the trial, we will evaluate for each patient seperately, whether seizures were less or more frequent during cannabidiol periods or placebo periods, to see what works best for every patient seperately. Participants will: * Register their seizures in a digital application during 4 to 6 periods. During these periods, CBD oil of placebo will be provided * Take CBD oil during 2 to 3 periods. One period will last around 7 weeks. * Also take placebo oil during the other 2 to 3 periods * Have a blood test every 7 weeks to monitor for safety and possible adverse effects * Visit the clinic once every 15 weeks for checkups and tests * Fill in questionnaires about quality of life, functioning, sleep, behaviour, epilepsy severity every 7 weeks.