Mine were 4 and 6 first time I took them. Definitely the MK! If you aren't a seasoned WDW veteran, I would suggest a guide book to do a bit of planning. Maybe even the WDW for Kids by Birnbaum. It make great bedtime reading for the kids, and will offer insight into which rides kids like. Other suggestions:
1) Get there early if you can. Breakfast at the Crystal Palace with Pooh and Friends is a nice way to start the day.
2) I still love the morning stroll down Main Street with the Castle in the backgroud! It really is Magical!
3) If you don't know about Fastpass, make sure you do by the time you get there. You'll be able to do so much more in one day, especially in June.
4) Ageless must do's for my kids (now 10 and 12) are the train, the "blue cars" (Tomorrowland Transit Authority), Carosel of Progress, Tomorrowland Speedway.
5) Take a group photo either by the Roy and Minnie Statue as you first get on Main Street, or by the Walt statue at the end of Main Street. Take the same photo every time you go back (assuming you will go back at some point).
6) Have a great trip!