My first visit my younger DD was 18 months and pretty lively, she was fine. Obviously the rides are not SUCH a big thing for them but she ADORED the parades, the nighttime Spectromagic parade @MK in particular. All the play areas that had water were popular, it wasn't open then but if you go to IOA the area in Seuss Landing is going to be a favourite I would guess. These little play areas are a great way for them to burn off excess energy while the water keeps them from overheating. SeaWorld has a great kids area and the digsite at AK is likely to keep them occupied for HOURS. Both BB and TL have great kiddie areas,
With a little one there are going to be a lot of things they can't do so I would suggest using these play areas to amuse the youngest while one parent takes the older off on thge "bigger rides" at 9 they are going to be able to get on most of the thrill rides. My older daughter used to REALLY like it when her sister wa too small to go on things because , when using the baby swaps they have on most rides, it allowed her to get 2 rides ( one with each parant) a lot of the times.
Character interaction can be a bit "hit and miss" at this age, IMHO it's important to let them take it at their own pace, being confronted by a 5 foot rodent can be a little traumatic at that age , even if he does own the place LOL. I've found a character breakfast gets them in the right frame of mind as the characters are well versed in noticing if a child is a little nervous about getting too close and they will take their time to make sure the kids ready. If you're just in a queue to have your photo taken with Mickey ( or whoever) everyone feels under pressure to get in and out as quickly as possible, this can result in a child whose not too sure about it being thrust into the arms of said rodent while they are not 100% sure about the transaction, end result is screams and tears.
Children at this age can get a massive amount out of a WDW trip, buit IMHO it's important to not rush them or overtire them. It does mean the most efficient way to do this is to split up the family at times and let the youngest have some fun on their own but the advent of fastpass has made this a great deal easier as you don't have to amuse them for a couple of hours while the older one queues for 90 minutes for Splash mountain.