Hi, WELCOME TO THE DIS!!!! Your 8 year old should be big enough to ride anything at WDW, and your 4 year old may be also. All of the coasters have height restictions, and they vary a little from ride to ride, but I don't know them all off the top of my head. The MK has the "mountain" rides, Space, Splash, and Big Thounder. MGM has RRC, and Epcot has TT and MS (not really a coaster.) You should have time to do lots with a 10 day pass. The weather in September will still be hot so plan to swim and try the water parks. Have fun planning...Lacee