When we were there in January, my family was getting ready to go inside Space Mountain. In front of us there was a dad and his 2 young DDs. The one DD could not ride due to her being too little. The other DD, who was a bit older, was the right height but looked to be about 5 or 6 years old (of course she could of been older but she was short so who knows exactly) and was able to go on the ride. The dad asked the CM if it was o.k. for the first DD to ride the ride without him and the CM said as long as the dad gave permission for her to ride alone then it is o.k. The dad gave his o.k. and then took the younger DD out of line and let the other DD walk the queue alone. I made sure DH and I kept an eye on the girl just in case she got confused, scared, or anything (she wasn't though). When we got to the loading area, the CM that put us in our designated spots didn't give a second look at her and let her get on the ride like normal. After the ride, we met back up with the girl again and had to show her which way was out. Luckily the girl's dad was standing at the exit (inside the shop) waiting for his DD. I was afraid that he might of been waiting somewhere outside, at which I would of helped her find her dad so she wouldn't get lost either.
Side note, if for some chance the girl was not going to be allowed to get on the ride because there wasn't an adult with her, I had already had it planned that I would explain the situation to that CM and then let them know is was o.k. that she could ride in the same car as us....didn't want her to be disappointed after all that.