The TNG episode in which Picard and several crewmembers become children is "Rascals." A transporter accident changes Picard, Guinan, Ro Laren and Keiko O'Brien into physically twelve-year-old versions of themselves while leaving their adult memories intact.
The transformation occurs as the four return to the Enterprise from Marlonia. Their sudden youth becomes a serious problem when Ferengi pirates seize the ship. Because the crew initially dismisses the transformed officers as children, Picard and the others must use ingenuity rather than rank to help retake the Enterprise.
Dr. Crusher develops a method to reverse the de-aging, but the crisis delays the treatment. Picard eventually uses a child's apparent tantrum as part of a deception that helps restore access to the ship's computer systems.
"Rascals" is the seventh episode of season six and originally aired in 1992. It is sometimes remembered as a Picard-only story, but Guinan, Ro and Keiko are transformed as well. Ro briefly chooses to remain young after the danger has passed.