My kids started playing about 5 or 6, now 8 and 10 they are good players and decent DMs. I homebrewed silly adventures for them when they were starting out. Ignore rules that cause confusion, they can be added back in later. Designing characters in DnDBeyond and minis in Heroforge was a big hit

The Dungeons & Dragons Young Adventurer's Guides gave them a bunch of ideas. They have read all of them cover to cover multiple times.