Sounds like a fantastic trip!
My girls were 5 and 6 on our last trip (WDW, Washington DC and NYC) and we all loved every minute of it
WDW tips:
If we didn't go back to the hotel for a midday break (which most people suggest), we had a sit down table service lunch which gave us a chance to rest in air conditioned comfort
You can't see everything in one trip, so let each person pick one attraction they 'must do/see' and everything else is gravy.
My girls loved spending time in the playgrounds (the boneyard and honey I shrunk the kids), it was a good chance for the adults to relax and stop the 'rush rush' that happens at WDW.
NYC tips:
Children under a certain height (think it might be 130cm?) can ride the subway free with a paying adult. The subway weekly pass is about $30 so great value!
An addition to the last trip - catch the subway! It is part of the experience and will save you walking so many blocks. We found the staff and other patrons very friendly and they would see hubby peering at the map and give us advice
Everyone recommends the Top of the Rocks so your photo of the NYC skyline can include the Empire Stare Building but we did the ESB because my 6 year old wanted to say she had been to the top. The view is magical
The museum of natural history is amazing and while they give you a suggested entry fee you can choose how much you pay.
I will keep thinking of more tips, the group on this board (the Aussie/NZ contingent) are so friendly and helpful.
My blog is in my signature if you want to have a look
