I also vote for the Essentials Kit. While the Starter Set adventure is good, it's easy to quickly have numerous plot threads and mysteries that players need to keep track of. That's probably too sophisticated for kids. In contrast, the Essentials Kit offers a simple premise ("A dragon is threatening your region. Protect it.") supported by short, linear quests with limited connections. Each quest in the adventure is 1 or 2 pages long. I think the longest is 4 pages. Most are good for about two hours of play. Depending on the speed at your table, obviously.