Unless the 3 year old is 40 inches, the 9mo old can do everything that the 3yr old can do. That is that either child can do anything with out a height restriciton. AT MK that is almost everything except the mountains and Alien Encounter. At MGM the only things I can think of that they can't do would be TOT, R&R and Star Tours . At AK they can do at lot too. They can't do dinosaur and the 9mo old not the Kali River ride but the 3 year old might can in the special child seat boat. At Epcot; TT and Body Wars would be out, as well as the new space ride if it is open by your trip. The truth is that the scare factor of some of the rides with out a height restriciton might bother your 3 yr old but a 9mo old that is too little for that fright. There might be more things she would go on than the 3 year old. Of course there are rides they will enjoy more than others but there is really not a lot you can not take them on with you. I am a big fan of Epcot for toddlers too as I find a lot of things there to interest little ones. There are also a lot of non ride things that your two little one will love. Parades, fountains, some shows, the light up sidewalks and just people watching. There will be more to do than you can get done.
Jordan's mom