Proper regulation of the cell cycle plays a fundamental role in any organism, as it impacts cellular division, differentiation and death. In this intricate process, progression through the phases of the cell cycle relies on various cyclin-cyclin-dependent kinase protein complexes that are regulated by multiple cyclin-dependent kinase inhibitors. Ultimately, the transcription factor E2F is influenced by this cascade and regulates the mRNA levels of proteins needed for cell cycle progression. Thus, disturbance of cell cycle regulation has been shown to be responsible for various types of cancer including adult and pediatric T-cell acute lymphoblastic leukemia and lymphoma (T-ALL and T-LBL, respectively), which are suggested to arise due to developmental arrest of precursor T lymphoblasts at certain early time points in T-cell maturation. Therapy optimization in recent years has resulted in good prognoses for patients of both malignancies. Here, we provide an overview of current knowledge of the cell cycle and its regulators with respect to pediatric T-ALL and T-LBL, both on molecular events underlying the disturbance of cell cycle regulation and on clinical parameters of affected children.