As stated in the Adventure Guides and Studio Guides:
All riders must be a minimum of 48" tall or accompanied by an adult on all attractions.
In other words, children must be at least 48" tall to use the single riders line. Children under 48" tall may not ride alone.
I can't tell you how many times I have witnessed parents attempting to put themselves and a 6 year old in a single rider line. Of course, on the ride the child will be safe. But what if the ride breaks down and has to be evacuated (it may be some time before the child is reunited with his/her parents - not to mention how scared the child may be, especially without their parent) - or what if the parent gets placed on the ride 5 minutes after the child (and the child is left wandering out the exit by themselves).
Use your best judgement - but many times the 48" height requirement is enforced in single rider lines such as Spiderman and River Adventure where the height requirement is much lower.