A Phase I Dose Finding Study in Children With Solid Tumors Recurrent or Refractory to Standard Therapy

Inclusion Criteria:

* Signed Informed Consent Form by subjects and/or subjects' parents/legal guardians and age appropriate Assent Form by the subjects obtained before any study specific procedure
* Age: from 6 months to less than 18 years old
* Diagnosis, Dose escalation phase of the study: subjects must have had histologic verification of solid malignancy at original diagnosis. Subjects with recurrent or refractory solid tumors are eligible, including primary central nervous system (CNS) tumors or subjects with known CNS metastases. Subject's current disease state must be one for which there is no known effective therapy or therapy proven to prolong survival with an acceptable quality of life. Effective therapy may include surgery, radiation therapy, chemotherapy or any combination of these modalities.

Dose expansion phase of the study: subjects must have relapsed/refractory RMS or a solid malignant tumor (Ewing sarcoma, hepatoblastoma, neuroblastoma and Wilms tumor) in which treatment with vincristine/irinotecan is considered backbone chemotherapy at relapse and a scientific rationale to combine vincristine/irinotecan with regorafenib exists.

* Subjects must have at least one measurable or evaluable lesion according to Response Evaluation Criteria in Solid Tumors (RECIST), version 1.1. For the neuroblastoma subjects with osteomedullary disease, the SIOPEN (International Society of Pediatric Oncology Europe Neuroblastoma Group) score will be used. Bone scans (if clinically indicated) should be obtained ≤12 weeks prior to the start of treatment.
* Life expectancy of at least 12 weeks from the time of signing informed consent/assent.
* Performance level: Karnofsky ≥ 70% for subjects \> 12 years of age or Lansky ≥ 70% for subjects ≤ 12 years of age
* Adequate hematological function assessed by the following laboratory requirements conducted within 7 days before starting study treatment:

Peripheral absolute neutrophil count (ANC): ≥ 1.0 x 10\*9/L Platelet count : ≥ 100 x 10\*9/L (transfusion independent) Hemoglobin: ≥ 8.0 g/dL

-Adequate hepatic function defined as:

* Aspartate aminotransferase/alanine aminotransferase (AST/ALT) ≤ 3.0\* ULN
* Bilirubin (sum of conjugated and unconjugated) ≤ 1.5 \* ULN

Exclusion Criteria:

* Prior treatment with regorafenib. Subjects permanently withdrawn from study participation will not be allowed to re-enter the study.
* Dose expansion phase of the study only: Subjects with brain tumors or subjects with known CNS metastases are excluded.
* Subjects with uncontrolled baseline hypertension higher than Grade 1 NCICTCAE v. 4.0
* Subjects with evidence or history of disorders of coagulation or thrombosis
* Cardiac abnormalities and cardiac arrhythmias requiring anti-arrhythmic therapy (beta blockers or digoxin are permitted)
* History of organ allograft (including allogeneic bone marrow transplant)
* Any other malignant disease treated prior to study entry
* Pregnancy or breast feeding
* Significant gastrointestinal disorders with diarrhea as a major symptom e.g., Crohn's disease or any malabsorption condition
* Close affiliation with the investigational site, e.g. a close relative of the investigator or a dependent person (e.g. employee or student of the investigational site)
* Unresolved toxicity higher than NCI-CTCAE v. 4.0 Grade 1 attributed to any prior therapy/procedure (excluding alopecia, chemotherapy-induced ototoxicity, Grade 2 chemotherapy-induced neuropathy and, as per above eligibility criteria, anemia with hemoglobin ≥ 8 mg/dL and ANC ≥ 1.0 x 10 9/L ).
* Any other malignant disease treated prior to the study