As Congress fought over the contours of the health care law six years ago, Sen. Robert Menendez won support for a plan to provide subsidized insurance for people under the age of 21 without the need to enroll an adult on the same policy. The idea was to provide a backstop for children with undocumented parents, those raised by grandparents and others who might otherwise slip through cracks in the social safety net.

Today, no one in government or the insurance industry has an accurate count on how many people are covered by “child-only” coverage. That makes it hard to assess whether there is enough outreach to families in need, if existing plans offer enough access to pediatric specialists and, more generally, whether the effort is even working.