Study participants will be screened during the platform study and randomly assigned to receive mirikizumab or another intervention. The purpose of the mirikizumab study is to evaluate efficacy, safety, tolerability, and how well mirikizumab absorbs into the body of pediatric participants with Crohn's disease. Study periods for the intervention-specific appendix (ISA) will be as follows: * A 12-week induction period * A maintenance period from Week 12 to Week 52, and * A safety follow-up period up to 16 weeks. The study will last about 74 weeks and may include up to 19 visits.