Additional Study Information: The purpose of this study is to investigate the long-term safety, effectiveness, and tolerability of Radiprodil, which is an investigational drug for the treatment of GRIN related Neurodevelopmental Disorder (GRIN-NDD). The study is broken up into 2 parts: Part A, which includes Screening, Medication Titration, and Maintenance periods and lasts 20-35 weeks, and Part B, which includes a 4-6 week transition titration period and an open label treatment period of at least 2 years. During Part A, half of the participants will receive Radiprodil, and half will receive a placebo (inactive) drug. Neither you nor the study team will know what treatment your child is receiving during this time. All of the patients in the study will receive Radiprodil during Part B of the study regardless of what they received during Part A. If you choose to participate, your child will need to visit the study site approximately 16 times within a 3-year period (7 times in Part A, 9 times in Part B), not including any additional unscheduled visits that may be needed. They will be evaluated for study inclusion, complete baseline assessments, begin taking the study drug twice per day every day, and perform follow-up assessments according to the study visit schedule.