I'm always up for a good challenge. I think it could be done, but you will have to have a lot of stamina and have a feel for what you are doing. I would try this by starting at MK on an am EMH day--you could easily hit all the mountains and be out of the park before 9am. Start with one adult going to get a FP at Space Mountain while the others head to Frontierland. Ride Splash, then BTMRR, head back to ride Space either do standby or use the Fast Pass. Next exit the park and head to EPCOT using the monorail. Get FP for Soarin (unless the standby line is 30 minutes or less then just get in line and ride) in the land Pavillion before heading to the TT and MS area. Ride whichever is shorter, grab a Fast pass for the other. If you still have the FP for soarin and can't get another yet, stand in line for whichever you didn't do yet. If you want to ride Maelstrom it shuld be open by now (11am) try that. Next head to AK (you'll need to use the bus for this), grab a fast pass for EE, Safari, Dinosaur or Kali. Check the standby times for Dinosaur, and Primeval Whirl (you can usually do one or the other while you wait for the FP to come due). This park is huge so be prepared to walk like a crazy person. ALways have at least one fast pass before standing on line for anything. You might want to grab a bite to eat between fast passes as well. Head over to MGM from AK (again you need the bus here). Once in MGM head over to TOT/RNR area and get a FP for one/do standby for the other. Once completed you might want to take in Star Tours. I doubt you'll have time to get the Lights, Motors, Action in, but it may be worth a try to check showtimes. You can usually get in standby as the theater is huge. So after checking showtimes it may be possible to see that and have a FP for Star Tours. By now you are going to be exhausted and hungry. If you wanted to cut a park out You might find getting to MGM from Epcot's International Gateway by boat a pretty good way to get where you want, but you definately will not have time to do all thrill rides AK has to offer since it closes earliest of all the parks. Another way to cut bus wait times down would be to take the resort buses if you could. What I am thinking here is that you could take the Boardwalk bus from AK to get you to MGM--just get off at BW and walk or boat to MGM. (If it were there and the MGM bus were not).
Honestly I'm not sure I would do this with a 9yo. Could it be done, yes. Is it a horrible way to experience these things, yes--you'll be on sensory overload and your enjoyment will not be optimum.
but possible. As dudspizza said I see one of the major obstacles being all the time wasted on Disney Transportation. 
