。
The lost child was crying quietly at the police box.
Literal
Lost child [subject-が] police-box [at-で] sob-sob was-crying.
しくしく is a mimetic specifically for quiet, soft, persistent crying — the kind of subdued sobbing a child does. Pairs with 泣く ('cry') in the past progressive ~ていた. 交番 is the quintessentially Japanese neighbourhood police box — much smaller than a police station, usually staffed by a single officer, and the default place to take a lost child.