A Study of Abemaciclib (LY2835219) in Combination With Other Anti-Cancer Treatments in Children and Young Adult Participants With Solid Tumors, Including Neuroblastoma

Inclusion Criteria:

* Parts A and B only:

  * Participants must be less than or equal to (≤)18 years of age.
  * Body weight greater than or equal to (≥)10 kilograms and body surface area (BSA) ≥0.5
  * Participants with any relapsed/refractory malignant solid tumor (excluding lymphoma), including central nervous system tumors, that have progressed on standard therapies.
  * For sites that are actively enrolling Parts B and C, participants with neuroblastoma who are eligible for Part C will be excluded from Part B unless approved by Lilly CRP/CRS.
* Part C only:

  * Participants must be less than (\<) 21 years of age.
  * Participants have a BSA ≥0.2 m².
  * Participants with first relapse/refractory neuroblastoma.
* All Parts

  * Participants must have measurable or evaluable disease by RECIST v1.1 or RANO.
  * A Lansky score ≥50 for participants \<16 years of age or Karnofsky score ≥50 for participants ≥16 years of age.
  * Participants must have discontinued all previous treatments for cancer or investigational agents and must have recovered from the acute effects to Grade ≤1 at the time of enrollment.
  * Able to swallow and/or have a gastric/nasogastric tube.
  * Adequate hematologic and organ function ≤2 weeks (14 days) prior to first dose of study drug.
  * Females of reproductive potential must have negative urine or serum pregnancy test at baseline (within 7 days prior to starting treatment).
  * Female participants of reproductive potential must agree to use highly effective contraceptive precautions during the trial. For abemaciclib, females should use contraception for at least 3 weeks following the last abemaciclib. For other study drugs, highly effective contraceptive precautions (and avoiding sperm donation) must be used according to their label.
  * Life expectancy of at least 8 weeks and able to complete at least 1 cycle of treatment.
  * Caregivers and participants willing to make themselves available for the duration of the trial.

Exclusion Criteria:

* Received allogenic bone marrow or solid organ transplant.
* Received live vaccination.
* Intolerability or hypersensitivity to any of the study treatments or its components.
* Diagnosed and/or treated additional malignancy within 3 years prior to enrollment that may affect the interpretation of results, with the exception of curatively treated basal cell carcinoma of the skin, squamous cell carcinoma of the skin, and/or curatively resected in situ cervical and/or breast cancers.
* Pregnant or breastfeeding.
* Active systemic infections.
* Serious and/or uncontrolled preexisting medical condition(s) that would preclude participation in this study.
* Parts A and C only: Have a bowel obstruction.
* Prior treatment with drugs known to be strong inhibitors or inducers of isoenzyme cytochrome P450 3A (CYP3A) or strong inhibitors of uridine diphosphate-glucuronosyl transferase 1A1 (UGT1A1) if the treatment cannot be discontinued or switched to a different medication at least 5 half-lives prior to starting study drug.
* Received prior treatment with cyclin-dependent kinase (CDK) 4 \& 6 inhibitor.
* Part C only: Received prior systemic therapy for relapsed/refractory neuroblastoma.
* Part C only, have received prior anti-GD2 therapy during induction phase.
* Currently enrolled in any other clinical study involving an investigational product or non-approved use of a drug or device.
* Has received an experimental treatment in a clinical trial within the last 30 days or 5 half-lives, whichever is longer.