Some of the country’s most prominent health insurance companies have decided to stop offering new child-only plans, rather than comply with rules in the new health-care law that will require such plans to start accepting children with preexisting medical conditions after Sept. 23.

The companies will continue to cover children who already have child-only policies. They will also accept children with preexisting conditions in new family policies.

Nonetheless, supporters of the new health-care law complain that the change amounts to an end run around one of the most prized consumer protections.