Medulloblastoma is a metastatic brain tumour arising in the cerebellum. It is the most common malignant brain tumour of childhood and occasionally occurs in adults. The most effective treatment is a combination of surgery, followed by radiotherapy and intensive cytotoxic chemotherapy; however, devastating long-term sequelae, due to the collateral damage of healthy tissue by these treatments, are frequently encountered by survivors. For patients with metastatic disease prognosis is dismal. Four molecularly distinct subgroups of medulloblastoma exist, with Group 3 recognised as the most aggressive and malignant subgroup. These patients have five-year overall survival probabilities ranging from 20-30%. When compared to overall survival rates of 80% across all medulloblastoma patients, this suggests current therapies are less effective in this subgroup. Many components of medulloblastoma therapy activate the DNA damage response pathway. Therefore, we hypothesised inhibition of this pathway may sensitise Group 3 medulloblastomas to treatment-induced cell death. In this study we evaluated inhibitors of cell cycle checkpoint kinases 1 and 2 (iCHKs) in multiple medulloblastoma models. All iCHKs tested reduced viability in a dose-dependent manner. To investigate their chemosensitising activity, medulloblastoma cells were treated with iCHKs in combination with the three DNA-damaging chemotherapeutics used in clinical treatment (cyclophosphamide, cisplatin, and gemcitabine). Based on mathematical modelling, potent synergy was observed in combination-treated cells using an alamar blue assay. Multiple iCHKs were further assessed in vivo using mice orthotopically implanted with human medulloblastoma cells. When combined with DNA-damaging chemotherapeutics, iCHK treatment reduced tumour burden as measured by bioluminescence imaging and significantly increased survival of tumour-bearing animals. Enhanced DNA damage and cytotoxicity was confirmed by increased γH2AX levels and caspase 3 cleavage. These data describe a robust preclinical approach to evaluate new therapies for paediatric medulloblastoma. Our findings strongly suggest that the combination of DNA damaging agents with iCHKs may improve current therapies and be a promising new treatment for this disease.