Dehydration is common and frequently under-diagnosed in chronic malnourished children, leading to life-threatening conditions. In this pilot study we applied bioimpedance vector analysis (BIVA) to determine hydration status in 52 neurologically impaired (NI) paediatric patients (14.08 ± 5.32). Clinical and biochemical data were used to define malnutrition and dehydration. Body composition analysis and hydration were also assessed by BIVA and we considered 143 normal-weight healthy subjects (15.0 ± 1.7), as controls for hydration status assessment. BIVA revealed a pathological hydration status in NI children, showing higher resistance (p < 0.001) and reactance values (p = 0.001) compared to controls. No differences in reactance and resistance were detected between well-nourished and under-nourished subjects. Four patients out of 52 showed mild signs of dehydration; no severe dehydration was detected. Laboratory data, suggestive for dehydration, were similar in well-nourished and under-nourished NI subjects. In conclusion, in our sample of NI paediatrics, dehydration according to clinical signs and laboratory data was under-diagnosed. BIVA showed specific bioelectrical characteristics that could be compatible with impaired hydration status. Further studies are necessary to confirm that BIVA may an applicable tool for defining dehydration status and guiding rehydration in NI children.