How to detect if a player has a certain child?

I am trying to make a ban script but i don't know how to check if a player has a certain child without the script outputting an error

Code:

game.Players.PlayerAdded:Connect(function(player)
	for _, v in pairs(game.Players:GetPlayers(player)) do
		local bool = Instance.new("BoolValue")
		bool.Name = ("Banned_" ..player.UserId)
		bool.Parent = player
		break
	end
end)

FindFirstChild is your friend here.

FindFirstChild will return true if the Instance exists or false if it does not.

e.g. if you wanted to check whether a Model called "Test" exists you would do:

print(workspace:FindFirstChild("Test"))