SPRINGFIELD — The state is moving ahead with its plan to shift an estimated 18,000 children from the child welfare system into a health-care coverage plan despite warnings from children’s advocates that the change will cut many of those children off from access to the critical care they need.“These are some of the most medically-complex and high-needs, special-needs children in the state,” Cook County Public Guardian Charles P. Golbert said during an interview. “These are children who by definition have been taken away …