How should I correct the second (underlined) sentence into a natural one?
Johnny's mother took him to a department store yesterday. His mother was not careful enough about him, so he was lost in the crowd.
(The other writing as follows: Johnny's mother took him to a department store yesterday. They walked a while together and then separated. In an unguarded moment, Johnny was lost in the crowd.)
She didn't watch him closely enough, and they got separated.