As PP noted, you must be 7 years old to ride alone at WDW. That being said, there are certain lines I would be more comfortable with DS9 going in alone than others. As AngiTN mentioned, you can see both the entrance and exit to Goofy's, so I would be comfortable with that. My concern is if a ride breaks down and they ask guests to leave the ride (we've experienced this at Space, Haunted Mansion and TT), I want to know where DS will be when he exits. For that reason, I still walk him through lines of rides I know I will not ride, but I can see him enter the ride and then go to the entrance so I'm there when he gets off. We do this at Space Mountain and EE (I burn my own FP+ for Space to do this, but I don't really care). On EE, I walk DS9 through the single rider line and then meet him at the exit of the ride. Your kids will be together, so probably not as much as an issue. I would, however, pick a designated meeting spot at the end of each ride where they should go when they exit OR if the ride happens to break down. Perhaps I'm overly cautious, but I don't particularly want to lose DS9 in WDW!