Because in March the ACA forbids you from continuing to cover your child under your health insurance policy and because the private health insurer will not allow you to change plans mid-cycle

http://forum.iwethey.org/forum/post/413805/

Now you're down to ...
I won't disagree that most insurance companies will allow you to switch plans after your youngest has his or her 26th birthday, but our broker has told us that a plain text reading of the law does not compel the carrier to do that.

So a plain reading does not compel them, according to your broker. This wouldn't by any chance be a broker you have selected because he agrees with your less-than-charitable views of the industry, would it?

You also aren't disputing my assertion that the clear intent of the law is that children must be covered at least until they are 26, and that their birthday is intended to be considered a qualifying event on the policy under which they're covered.