I have visited WDW and DL often with a mentally challenged son and young grandkids. It's a real worry. Often men's restrooms have more than one exit, so I'll be waiting at one door and my ds wanders out another door! Companion restrooms are a real blessing. My solution was to not overhydrate myself so that I didn't need to use the restrooms myself (I'm not recommending this, you understand, it just served the purpose at the time), and camped outside the men's room, frequently calling in to make sure he hadn't left via another door.
Thank heavens those days are over!
Thank heavens those days are over!