Advances in prenatal and neonatal care have improved outcomes in children with severe congenital heart disease (CHD). With the increase in survival, neurocognitive problems such as executive functioning (EF) impairments have become more apparent in these children. EF problems have cascading negative effects on a child's development. New insights in EF development suggest that in otherwise physically healthy young children, EF can be improved by training. In a pilot study funded by Stichting Hartekind, the investigators studied the feasibility of a personalized EF training program called 'Kleuter Extra' and the results were promising. Therefore, the current study will investigate the effectiveness of this program in 4-6-year-old children with severe CHD. The researchers will also explore interactions between the parent-child relationship and EF development of the child as psychosocial difficulties in these children and their parent(s) and/or caretaker(s) may impact EF-development. If found effective, EF training for children with severe CHD will improve their developmental outcome.