Autoincrementing variable in CloudFormation

child.json

{
    "Outputs": {
        "Value": {
            "Value": {
                "Ref": "ParameterValue"
            }
        }
    },
    "Parameters": {
        "ParameterName": {
            "Type": "AWS::SSM::Parameter::Name"
        },
        "ParameterValue": {
            "Type": "AWS::SSM::Parameter::Value<String>"
        }
    },
    "Resources": {
        "Increment": {
            "DependsOn": [
                "IncrementLogGroup",
                "IncrementPolicy"
            ],
            "Properties": {
                "ParameterName": {
                    "Ref": "ParameterName"
                },
                "ParameterValue": {
                    "Ref": "ParameterValue"
                },
                "ServiceToken": {
                    "Fn::GetAtt": [
                        "IncrementFunction",
                        "Arn"
                    ]
                }
            },
            "Type": "AWS::CloudFormation::CustomResource"
        },
        "IncrementFunction": {
            "Properties": {
                "Code": {
                    "ZipFile": "def handler(event, context):\n    import boto3\n    import json\n    import urllib.parse\n    import urllib.request\n\n    print(\"Request:\", json.dumps(event))\n\n    resource_response = {\n        \"Status\": \"SUCCESS\",\n        \"PhysicalResourceId\": (\n            event[\"ResourceProperties\"].get(\"ParameterName\") or\n            event.get(\"PhysicalResourceId\")\n        ),\n        \"StackId\": event[\"StackId\"],\n        \"RequestId\": event[\"RequestId\"],\n        \"LogicalResourceId\": event[\"LogicalResourceId\"],\n    }\n\n    try:\n        ssm = boto3.client(\"ssm\")\n        if event[\"RequestType\"] in {\"Create\", \"Update\"}:\n            next_value = str(int(event[\"ResourceProperties\"][\"ParameterValue\"]) + 1)\n            ssm.put_parameter(\n                Name=event[\"ResourceProperties\"][\"ParameterName\"],\n                Value=next_value,\n                Type=\"String\",\n                Overwrite=True,\n            )\n    except Exception as ex:\n        resource_response.update({\n            \"Status\": \"FAILED\",\n            \"Reason\": f\"{ex.__class__.__qualname__}: {ex}\"\n        })\n\n    print(\"Response:\", json.dumps(resource_response))\n\n    response = urllib.request.urlopen(\n        urllib.request.Request(\n            url=event[\"ResponseURL\"],\n            data=json.dumps(resource_response).encode(\"utf-8\"),\n            method=\"PUT\",\n        )\n    )\n"
                },
                "Handler": "index.handler",
                "MemorySize": 512,
                "Role": {
                    "Fn::GetAtt": [
                        "IncrementRole",
                        "Arn"
                    ]
                },
                "Runtime": "python3.7"
            },
            "Type": "AWS::Lambda::Function"
        },
        "IncrementLogGroup": {
            "Properties": {
                "LogGroupName": {
                    "Fn::Join": [
                        "/",
                        [
                            "/aws/lambda",
                            {
                                "Ref": "IncrementFunction"
                            }
                        ]
                    ]
                }
            },
            "Type": "AWS::Logs::LogGroup"
        },
        "IncrementPolicy": {
            "Properties": {
                "PolicyDocument": {
                    "Statement": [
                        {
                            "Action": [
                                "ssm:PutParameter"
                            ],
                            "Effect": "Allow",
                            "Resource": [
                                {
                                    "Fn::Join": [
                                        ":",
                                        [
                                            "arn",
                                            {
                                                "Ref": "AWS::Partition"
                                            },
                                            "ssm",
                                            {
                                                "Ref": "AWS::Region"
                                            },
                                            {
                                                "Ref": "AWS::AccountId"
                                            },
                                            {
                                                "Fn::Join": [
                                                    "/",
                                                    [
                                                        "parameter",
                                                        {
                                                            "Ref": "ParameterName"
                                                        }
                                                    ]
                                                ]
                                            }
                                        ]
                                    ]
                                }
                            ]
                        },
                        {
                            "Action": [
                                "logs:CreateLogStream",
                                "logs:PutLogEvents"
                            ],
                            "Effect": "Allow",
                            "Resource": [
                                {
                                    "Fn::GetAtt": [
                                        "IncrementLogGroup",
                                        "Arn"
                                    ]
                                }
                            ]
                        }
                    ]
                },
                "PolicyName": "IncrementPolicy",
                "Roles": [
                    {
                        "Ref": "IncrementRole"
                    }
                ]
            },
            "Type": "AWS::IAM::Policy"
        },
        "IncrementRole": {
            "Properties": {
                "AssumeRolePolicyDocument": {
                    "Statement": [
                        {
                            "Action": [
                                "sts:AssumeRole"
                            ],
                            "Effect": "Allow",
                            "Principal": {
                                "Service": "lambda.amazonaws.com"
                            }
                        }
                    ]
                }
            },
            "Type": "AWS::IAM::Role"
        }
    }
}

parent.json

{
    "Outputs": {
        "Value": {
            "Value": {
                "Fn::GetAtt": [
                    "Stack",
                    "Outputs.Value"
                ]
            }
        }
    },
    "Resources": {
        "Parameter": {
            "Properties": {
                "Tier": "Standard",
                "Type": "String",
                "Value": "1"
            },
            "Type": "AWS::SSM::Parameter"
        },
        "Stack": {
            "Properties": {
                "Parameters": {
                    "ParameterName": {
                        "Ref": "Parameter"
                    },
                    "ParameterValue": {
                        "Ref": "Parameter"
                    }
                },
                "TemplateURL": "/path/to/child.json"
            },
            "Type": "AWS::CloudFormation::Stack"
        }
    }
}