redditPost.swift

import UIKit

let json = """
{
    "kind": "Listing",
    "data": {
        "modhash": "asjdfhhjsdfkjhsdfksjd23423",
        "dist": 25,
        "children": [
            {
                "kind": "t3",
                "data": {
                    "title": "Test title",
                    "score": 88
                }
            }
        ]
    }
}
"""

let data = json.data(using: .utf8)

struct RedditPost: Decodable {
    let title: String
    let score: Int

    enum CodingKeys: String, CodingKey {
        case data
    }

    enum RedditPostKeys: String, CodingKey {
        case title
        case score
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let innerContainer = try container.nestedContainer(keyedBy: RedditPostKeys.self, forKey: .data)
        self.title = try innerContainer.decode(String.self, forKey: .title)
        self.score = try innerContainer.decode(Int.self, forKey: .score)
    }
}

struct RedditPostListing: Decodable {
    enum CodingKeys: String, CodingKey {
        case data // you only need to assign a string value here if the value is different
    }

    enum ChildrenCodingKeys: String, CodingKey {
        case children
    }

    let children: [RedditPost]

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let data = try container.nestedContainer(keyedBy: ChildrenCodingKeys.self, forKey: .data)
        self.children = try data.decode([RedditPost].self, forKey: .children)
    }
}


let decoder = JSONDecoder()
let listing = try decoder.decode(RedditPostListing.self, from: data!)

print(listing.children)