Brother, Can You Spare a Crime by Sheri Cobb South

When one of his colleagues witnesses a robbery in which one of the participants, a child, bears a striking resemblance to a certain Bow Street Runner, John Pickett realizes he has a young half-brother who has been apprenticed to a criminal gang. The brutal murder of the pursuing constable makes the crime far more serious than a simple robbery would be-and ten-year-old Kit is linked to the crime by the toy soldier he dropped while fleeing the scene.
In spite of his tender years, he will certainly hang unless Pickett can extricate him before fellow Bow Street Runner Robert Maxwell solves the case. For the first time in his career, Pickett is obliged to work against one of his fellows, dogging Maxwell's steps and trying to anticipate his next move in order to get to the boy first.
Pickett's investigation will take him back to the rookeries of London where his own childhood was spent. But can he return to his old haunts without being pulled back into his old way...

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