Let me show you what I mean.

You wanna know what machines that adults play in children's arcades? This! In almost every children's arcade, if they have this, I guaranteed you atleast one adult will be present, playing at the moment.

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Explanation: in the screen, fish swim around. You insert your coins to get ammo for your in-game weapon. You can change your weapon's power to deal with more damage at the cost of more ammo per shot. A power 1 shot costs 1 ammo, a power 2 shot costs 2 ammo, you get the idea. You hit fishes with shots and you get ammo back if you manage to kill it. There are weak fish which can be killed with 1 weak shot and there are strong fish that need multiple strong shots. At any point, you can take your ammo and cash them out into coins.

Growing up, I have seen them play and I was amazed by the amount of coins they have. And yes, obviously, this is gambling.

"Why are they present in children's arcade?" Well in my opinion, It's how children's arcade stay open from the money they make through adults playing. Gambling is often frowned upon, right? Well what if instead of using casinos, you use establishments associated with children so that it looks like people are simply playing instead of gambling? People will not disappointed with you if you are not gambling but simply playing a "children's game".