The non-invasive assessment of resting-state (RS) neural activity using electroencephalography (EEG) or magnetoencephalography (MEG) provides information on brain function (state of arousal, local and long-range functional connectivity) and brain health (in typically developing individuals and individuals with a neurodevelopmental diagnosis). Our understanding of RS neural activity in older children and adults is limited by our poor understanding of the maturation of RS activity from infancy to preschool ages. Scientists examining the maturation of RS neural activity often study the dominant oscillation, referred to in older children and adults as the peak alpha frequency and in infants as the infant dominant frequency. The present study describes the maturation of RS activity in a cohort of 100+ children 2-68 months of age, using an eyes-open Dark-Room RS paradigm optimal for assessing oscillatory RS activity in infants and young children: alternating 5 times between a Dark-Room (30 s) and a Video-On (20 s) condition. Distributed source modeling measured parietal-occipital RS periodic activity, with analyses examining the maturation of the RS infant dominant frequency. The parietal-occipital infant dominant frequency increased nonlinearly as a function of age, with rapid maturation from birth to 2 years, and with continued, but slower, changes thereafter. Comparison of Dark-Room and Video-On RS measures showed that the infant dominant frequency was more often observed, and of higher strength, in the Dark-Room than Video-On condition. Present findings demonstrate the use of an eyes-open Dark-Room task provides measures of young child RS periodic activity with an excellent signal-to-noise ratio.