I did this with my friend's kids, boy age 7, girl age 4. My friend didn't want to go and the kids were dying to go back. Their ages were too spaced out to take them together, as she was too little to ride the "wild" rides and I couldn't very well send him on the rides alone while I waited with her.
I took her first, in September for 4 nights. We had a GREAT time! Took everything at her pace. I had made him a Disney Countdown Calendar on the computer where he tore off a page a day, so he knew how long until his trip. I gave it to him the day we left on her trip.
His trip was in December and we stayed 5 nights. Again, had a BLAST! We went to Disney Quest, too. Those are 2 of the best trips I've ever had!