Sequential PD-1/PD-L1 Inhibitor and LENvatinib in TLCT and Refractory Hepatoblastoma After Chemotherapy

Inclusion Criteria:

* Patients must be \>= than 5 years and =\< 14 years of age at the time of study enrollment
* pathological diagnosis of TLCT/NOS, or hepatoblastoma（HB）（Emergent Treatment for HB In emergency situation when a patient meets all other eligibility criteria and has had baseline required observations, but is too ill to undergo a biopsy safely, the patient may be enrolled without a biopsy.）
* Failed prior first-line or second-line chemotherapy
* general charactoristics: Lansky performance status 50-100% in patients ≤ 10 years of age OR Karnofsky performance status 50-100% in patients \> 10 years of age Life expectancy \> 8 weeks Hemoglobin \> 8 g/dL Absolute neutrophil count \> 1,000/mm\^3 Platelet count \> 100,000/mm\^3 Total bilirubin ≤ 5 x upper limit of normal (ULN) for age, Aspartate aminotransferase (AST) or Alanine transaminase (ALT) \< 10 x upper limit of normal (ULN) for age. Serum creatinine ≤ 3 times normal Normal metabolic parameters (i.e., serum electrolytes, glucose, calcium, and phosphate) Not pregnant or nursing No severe uncontrolled infection or enterocolitis
* Recovered from toxicity of prior therapy No chemotherapy within 3 weeks prior to study entry No prior PD1/PD-L1 blockade treatment

Exclusion Criteria:

* Concurrent enrollment in another clinical study, unless it is an observational (non-interventional) clinical study or during the follow-up period of an interventional study.
* Any unresolved toxicity NCI CTCAE Grade ≥2 from previous anticancer therapy with the exception of alopecia, vitiligo, and the laboratory values defined in the inclusion criteria.
* Patients who are currently receiving another investigational drug.
* Patients who are currently receiving other anticancer agents.
* Patients with uncontrolled infection.
* Current or prior use of immunosuppressive medication within 14 days before the first dose of PD1/PD-L1 blockade