I think 12 and 11 are reasonable ages for what you are proposing. That said, if you are on the cusp, then I would try doing things in slightly smaller and more controlled doses. Rather than giving them complete freedom for a couple hours, you could do things in "sections."
It would look a bit different in different parks, but using AK as an example:
Say you are in Dinoland - you could send to two older kids to ride Dinosaur and Primeval Whirl, while you and the 2 year old go several rounds on Triceratop Spin. This way, you are all in the same area, but everyone gets to have fun. For the most part, every "thrill" ride has something nearby that a little one would enjoy doing.
And don't forget about baby swap, your 11 and 12 year olds might be more inclined to want to stay together when they realize that having a baby along can get them extra turns on their favorite rides.
That said, while there are certainly some rides that the 2 year old won't be able to go on, there are very few "kiddie" rides at WDW. Most are designed to be enjoyed by the whole family. I set a rule with my kids many years ago that you are not allowed to complain about others choices and unless you are scared, you ride and are happy about it. (or acting happy - ride It's a Small World with my teenage boys and you will hear them gleefully exclaim their delight...

)