Electroencephalographic density spectral array monitoring has been developed to facilitate the interpretation of unprocessed electroencephalogram signals. The primary aim of this prospective observational study, performed in a tertiary children's hospital, was to identify the clinical applicability and validity of density spectral array monitoring in infants and children during sevoflurane anaesthesia. We included 104 children, aged < 6 years, undergoing elective surgery during sevoflurane anaesthesia. We investigated the correlation between non-steady state end-tidal sevoflurane and the expression of the four electroencephalogram frequency bands β, α, θ and δ, representing density spectral array. Patients were divided into three age groups (< 6 months, 6-12 months, > 12 months). There was a significant correlation between end-tidal sevoflurane and density spectral array in the age groups 6-12 months (p < 0.05) and 1-6 years (p < 0.0001). In infants < 6 months of age, the relative percentages of density spectral array did not correlate with end-tidal sevoflurane. The main finding was that different end-tidal concentrations of sevoflurane produce age-dependent changes in the density spectral array power spectrum. In infants younger than 6 months-old, α and β coherence are absent, whereas θ and δ oscillations have already emerged. In cases where anaesthesia was too deep, this presented as burst suppression on the electroencephalogram, θ disappeared, leaving the electroencephalographic activity in the δ range. Future research should address this issue, aiming to clarify whether the emergence of θ oscillations in infants helps to prevent sevoflurane overdosing.