This study examines the costs and service use for children enrolled in public health insurance in San Mateo County, California using encounter data from the Health Plan of San Mateo. In that county, the financing burden for childrens public health insurance programs is concentrated in the top tenth of health care users, while the other 90% of children are very inexpensive. This finding suggests that early identification and greater efficiency in care management for high-cost children could free up funding to expand public insurance services to more low-cost children. As states evaluate the feasibility of public health care for all children, it is important to examine the distribution of costs and services within existing programs, in order to better plan services for the highest cost children.