Tazdeb has given you some great advice. What I'd like to add is for you to consider booking a character breakfast at Cinderella's Royal Table in Magic Kingdom. It's a pain to do...it's the most popular priority seating in WDW...but if your daughters are into the princesses, you just can't beat it. Our 4 yr. old daughter and 2yr. old son loved it, as they watch many of the Disney Classic movies.
For your trip, I would suggest sticking with WDW. It can be overwhelming to both adults and children and you should build in some downtime to rest and relax. Preschool age is not too young for WDW, but it is young for "commando-style" park touring. I think we're able to tour successfully with our children because we pay attention to their signals. sometimes it means leaving a park earlier than we planned. At other times, we've spent a full day at MK without naps and without meltdowns later!
Another thing to keep in mind, it's sometimes unpredictable how children will react to the characters. Some love them, some are very scared. Mine are scared of the costumed characters who wear headpieces, but can handle the costumed characters at Cindy's breakfast.
You'll see the parks a whole new way through a preschoolers' eyes!
Sharon