Howdy Karin!
With regards to dining without reservations, I'm focusing mainly on quick service locations as table service dining without reservations can be challenging at times and I know you don't want to have a lengthy wait with three little ones. If you can, try to avoid the peak meal times so you can enjoy smaller crowds.
Disney's BoardWalk: The Pizza Window is a great place to get a slice and a beverage. There is typically little wait time and you can enjoy your meal while taking in the sights and sounds of the BoardWalk.
Magic Kingdom: one of our favorite quick service locations is Cosmic Ray's Starlight Café at Magic Kingdom. They have several choices that will satisfy the pickiest of eaters and alien Audio-Animatronics entertainer, Sonny Eclipse, will keep your kiddos entertained while the other adults select the meals.
Epcot has numerous dining options! At Epcot, we have been able to dine at both Rose & Crown and Biergarten Restaurants without reservations. Biergarten offers a buffet and is one of my kiddos favorite restaurants due to the year long Oktoberfest celebration and entertainment. For quick service, we enjoy Sunshine Seasons in The Land.
At Disney's Hollywood Studios, we enjoy Backlot Express and while at Animal Kingdom, we enjoy Flame Tree Barbecue and Restaurantosaurus.
Regarding
Star Wars Weekend: it ends on June 9 this year so be sure to visit Disney's Hollywood Studios your 1st or 2nd day of vacation. We were at opening weekend in 2012 and my then 3 year old became a Star Wars fanatic! There is a special parade, Hyperspace Hoopla, celebrity autographs and more. You will not want to miss it!!
Please let me know if you need further information and have a magical day!!