First things first: “Don’t panic,” said Stephanie Lee, Psy.D., senior director of the A.D.H.D. and Behavior Disorders Center at the nonprofit Child Mind Institute in Manhattan. Stay calm in the face of regressive behavior, because freaking out doesn’t help anyone. Then try to figure out what might be triggering the behavior. Could it be that your kid is going through a tough transition, or is experiencing a big routine change? Are you super-stressed and your kid is picking up on it? Or did your child maybe just accomplish a big developmental feat?