I remember reading on the boards that a mom was allowed to bring her son on the aqua duck with her only because she was going too. She also noted that it may not be the same rule for all the employees who work the loading area but knowing that maybe you have a chance. I have an almost 4 year old coming with me and he wants to ride it too but I think I'll be able to sway him to nemos reef.
My son, 11 years old, had to have a parent go on the ride because he was too light and kept getting stuck. My younger kids, 7 and 5, rode with their cousins, 10 and 11.