Detailed assessment of exposure to extremely low frequency (ELF) and intermediate frequency (IF) fields is essential in order to conduct informative epidemiological studies of the health effects from exposure to these fields. There is limited information available regarding ELF electric fields and on both magnetic and electric field exposures of children in the IF range. The aim of this study was to characterize ELF and IF exposure of children in the Spanish INMA cohort. A combination of spot and fixed measurements was carried out in 104 homes, 26 schools and their playgrounds and 105 parks. Low levels of ELF magnetic fields (ELF-MF) were observed (with the highest 24-h time-weighted average (TWA) exposure being 0.15 μT in one home). The interquartile range (IQR) of ELF electric fields (ELF-EF) ranged from 1 to 15 V/m indoors and from 0.3 to 1.1 V/m outdoors and a maximum value observed was 55.5 V/m in one school playground. IQR ranges for IF magnetic and electric fields were between 0.02 and 0.23 μT and 0.2 and 0.5 V/m respectively and maximum values were 0.03 μT and 1.51 V/m in homes. Correlations between magnetic and electric fields were weak for ELF (Spearman 0.04–0.36 in different settings) and moderate for IF (between 0.28 and 0.75). Children of INMA-Gipuzkoa cohort were exposed to very low levels of ELF-MF in all settings and to similar levels of ELF-EF compared to the range of previously reported levels, although somewhat higher exposures occurred at home. Children enrolled to our study were similarly exposed to IF in all settings.