I would say that an ipad for learning is 'good to have', but not essential at this stage.
No, you can't run Windows programs on it, but you can use programs like Splashtop Remote to remotely view your PC:
http://forum.brillkids.com/little-reader/little-reader-available-on-the-ipad-today!/
I think for the vast majority of useful educational programs for little kids on the iPad, there will be substitutes to be found on the PC. What is difficult to replace are those requiring tactile feedback or control, like:
- drawing apps (but then there's always pen/paper...)
- apps like Tap Tap where you tap on the iPad along with the song (good for training rhythm and hand-eye coordination).