Thousands are bypassing age checks to install it from Apple or Google stores.
Police fear Cunch-Line Chronicles could inspire children to be pushers.
The Mario-style game mimics county lines rackets – cunch in slang.
Players dodge cops and collect drugs.
Grace Groom, whose son Sam Mechelewski, 20, was murdered by dealers in Cambridgeshire in 2018, said: “What they’re doing is really dangerous.
“County lines are not funny – they use rape and violence to exploit children.”
Dr Simon Harding, of the National Centre for Gang Research at West London University, called the game “a form of grooming or recruitment”.
He said: “It normalises the crime and could put young people at risk.
“It makes them feel that county lines is no longer hidden – you can even get a gaming app.”
The National Crime Agency said it was aware of the game and was looking at ways to take action.
It says county lines gangs make £500million a year by exploiting kids as young as seven to sell drugs.
Game maker David Olaleye, 23, said he “knew children would play the game” but dismissed it as a problem.
The software boss said: “I don’t think it glamorises violence. I was inspired by other games, like Grand Theft Auto.”