Hi Mary,
Power Rangers made their last appearance in Hollywood Studios a couple of years ago, so you won't be able to find them on your next vacation. There are a lot of other characters that hopefully your son will be interested in meeting: Phineas & Ferb, Buzz & Woody, Incredibles, and Monsters, Inc. - just to name a few.
As for Winnie the Pooh, your daughter will have many opportunities to meet him. The loveable bear can be found in every park. Just check the park guide when you arrive for specific locations. He also has his own character meal at Crystal Palace in Magic Kingdom, and he and Tigger are part of the 1900 Park Fare character breakfast at Grand Floridian. In Magic Kingdom, Winnie the Pooh has his own attraction,
The Many Adventures of Winne the Pooh. I think it's safe to say that your daughter will get to see her favorite Disney character at least once.
Hope this helps. Have a magical vacation!