A whole-head magnetoencephalography (MEG) system was developed to study cognitive processing in young children. The child MEG system has a helmet-shaped sensor array designed to fit child head sizes. The sensor array is composed of 64 LTS-SQUID axial-type gradiometric magnetometers with 50 mm of baseline length, arranged about 100 mm from the center of the child's head. The sensor array is installed in a helmet of a horizontal dewar with a head circumference of about 530 mm, which was determined on the basis of the preliminary investigation of the standard pre-school children's head. The liquid helium capacity of the dewar is roughly 100 liters, and the helium consumption rate is less than 6 liters/day. The sensors have been positioned in the dewar using a ship-in-a-bottle approach. To verify the performance of the child MEG system, an auditory evoked field measurement was taken of a healthy 4-year-old child subject. Large simultaneous magnetic field components corresponding to the P100m were successfully observed in the child over both the right and the left hemispheres. The latency of the effect was at around 130 ms, and two equivalent current dipoles were found in the temporal lobes in both hemispheres of the child subject.