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Simple Summary: Patients with pediatric liver cancers hepatoblastoma and hepatocellular carcinoma very often develop lung metastases. These cancers can present with lung metastases and are at higher risk of relapse. Although cisplatin is very effective at clearing lung metastases, they can still relapse. Therefore, there is an urgent need to develop therapeutic approaches to prevent the development of lung metastases in patients with pediatric liver cancers. In this paper, we show that the metastatic microenvironment of HBL and HCC patients contains a heterogeneous cell population that formed tumor clusters. We found that both fresh primary tumors and generated primary cell cultures had increased the expression of HDAC1, a histone deacetylase, and the transcription factor Sp5. Sp5 and HDAC1 work in tandem by transporting HDAC1 to the promoters of genes and changing their expression. We analyzed the effects of the HDAC inhibitor, SAHA, on the metastasis-initiating cells in combination with cisplatin. We found that HDAC inhibition increases the efficacy of cisplatin to eliminate these metastasis-initiating cells. The pediatric liver cancers, hepatoblastoma and hepatocellular carcinoma, are dangerous cancers which often spread to the lungs. Although treatments with cisplatin significantly improve outcomes, cisplatin may not eliminate metastasis-initiating cells. Our group has recently shown that the metastatic microenvironments of hepatoblastoma contain Cancer Associated Fibroblasts (CAFs) and neuron-like cells, which initiate cancer spread from liver to lungs. In this study, we found that these cells express high levels of HDAC1; therefore, we examined if histone deacetylase inhibition improves cisplatin anti-proliferative effects and reduces the formation of tumor clusters in pediatric liver cancer metastatic microenvironments. Methods: New cell lines were generated from primary hepatoblastoma liver tumors (hbl) and lung metastases (LM) of HBL patients. In addition, cell lines were generated from hepatocellular neoplasm, not otherwise specified (HCN-NOS) tumor samples, and hcc cell lines. Hbl, LM and hcc cells were treated with cisplatin, SAHA or in combination. The effect of these drugs on the number of cells, formation of tumor clusters and HDAC1-Sp5-p21 axis were examined. Results: Both HBL and HCC tissue specimens have increased HDAC1-Sp5 pathway activation, recapitulated in cell lines generated from the tumors. HDAC inhibition with vorinostat (SAHA) increases cisplatin efficacy to eliminate CAFs in hbl and in hcc cell lines. Although the neuron-like cells survive the combined treatments, proliferation was inhibited. Notably, combining SAHA with cisplatin overcame cisplatin resistance in an LM cell line from an aggressive case with multiple metastases. Underlying mechanisms of this enhanced inhibition include suppression of the HDAC1-Sp5 pathway and elevation of an inhibitor of proliferation p21. Similar findings were found with gemcitabine treatments suggesting that elimination of proliferative CAFs cells is a key event in the inhibition of mitotic microenvironment. Conclusions: Our studies demonstrate the synergistic benefits of HDAC inhibition and cisplatin to eliminate metastasis-initiating cells in pediatric liver cancers. [ABSTRACT FROM AUTHOR]