In this study, researchers will learn more about the study drugs diroximel fumarate (DRF) and dimethyl fumarate (DMF) in children with MS who may be experiencing relapses. Participants will be divided into 2 groups based on their weight: * Group A will include children who weigh 40 kilograms (kg) or less. They will receive DRF in both Part 1 and Part 2 of the study. * Group B will include children who weigh more than 40 kg. They will be randomly assigned to receive DRF, DMF, or fingolimod. Fingolimod is a drug already used to treat MS in adults. In Part 2, they will receive DRF. This is a 2-part study. Part 1 treatment will last 96 weeks. Participants who complete Part 1 may move to Part 2. Part 2 is an extension period. Treatment will last another 96 weeks and will help researchers learn about the long-term safety and effects of treatment. The main goal of the study is to learn about the safety of DRF and DMF and compare their effect on relapses and brain lesions with fingolimod. The main questions researchers want to answer are: * How many participants have adverse events and serious adverse events? * How often do participants relapse after treatment with DRF and DMF compared to fingolimod? Researchers will take brain imaging scans to check for any new areas of brain inflammation and compare the brain lesions before and after treatment. Researchers will also measure the amount of drug in the blood to understand how the body processes it. To check safety, they will monitor participants' growth and development, and compare changes in heart tests, vital signs, and lab tests. They will also use rating scales to monitor depression symptoms. The study will be done as follows: Part 1 (Treatment Period) * After screening, participants will join Part 1 and be divided into 2 groups based on their weight. * Participants in Group A will receive DRF. * Participants in Group B will be randomly assigned to receive either DRF, DMF, or fingolimod. * Neither the researchers nor the participants will know which drug or dose the participants will receive in Group B. Participants in Group B will also receive a placebo so they do not know which drug is being given. A placebo looks like a study drug but contains no real medicine. * All study drugs will be taken by mouth, once or twice a day. Participants who take DRF or DMF will start with a lower dose during the 1st week, then move to a standard dose. * Treatment in Part 1 will last for 96 weeks. Participants will have up to 11 study visits and 7 phone calls. * Participants who do not move onto Part 2 will also have a safety follow-up period of 4 to 8 weeks. This will include 1 study visit and 1 phone call. * The total length of Part 1, including the screening, treatment, and follow-up, will be up to 108 weeks. Part 2 (Extension Period) * Participants who complete Part 1 can move on to Part 2 of the study. * All participants in Part 2 will receive DRF by mouth for 96 weeks. * Participants will have up to 10 more study visits and 1 telephone call during treatment. * They will also have a 4-week safety follow-up, including 1 study visit and 1 phone call. * The total length of Part 2 will be up to 100 weeks.
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TP Cohort A and OLE Period: Number of Participants With Treatment Emergent Adverse Events (TEAEs), Serious Adverse Events (SAEs) and AEs Leading to Treatment Discontinuation
Timeframe: Cohort A: From first dose of study drug up to end of follow-up (up to Week 104), OLE: Baseline (Week 96) up to the end of OLE period (up to Week 196)
TP Cohort B: Annualized Relapse Rate (ARR) Through Week 96
Timeframe: Baseline (Day 1) up to Week 96