Terraform pass variables to a child module

James is correct but his syntax will work just for string variables. With others you have to specify the 'blank default':

variable "passed_list_var" {
    default = []
}

variable "passed_map_var" {
    default = {}
}

You need to declare any variables that a module uses at the module level itself:

variable "account_id" {
}

resource "aws_iam_policy_attachment" "myName" {
    name       = "myName"
    policy_arn = "arn:aws:iam::${var.account_id}:policy/myName"
    groups     = []
    users      = []
    roles      = []
}