It depends what ride you are talking about. I'm assuming that you wouldn't want your toddler to go on any ride alone.....I don't think there are any rides at WDW that a toddler would ride alone. If there is a height requirement for a ride, and your toddler does not meet that, they cannot ride - with an adult or not. It is a question of safety systems, not supervision or appropriateness. There are many rides without a height requirement that you can enjoy with your toddler, many websites have a list of all height requirements, and the rides not on that list are open range. If someone can offer a better answer feel free - or correct me if I am mistaken!!