To get lost; to become separated or unable to find one's way (often used for children or people in unfamiliar places).
駅で小さな子が迷子になってしまい、スタッフに預けられた。
A small child got lost at the station and was taken to the staff.
旅行中に道に迷って、しばらく迷子になった。
While traveling I got turned around and was lost for a while.
テーマパークでは子どもが迷子にならないよう、連絡先を持たせておくと安心だ。
In theme parks it's reassuring to give children contact details so they don't get lost.