Safari is the best ride for the whole family. Last trip we did it at rope drop and again with a FP in the afternoon, just to see what differences there might be in which animals were out and active.
As PPs have said, the boneyard is great for little kids. I loved Dinosaur, and your 6-year-old might enjoy it, too. Maybe have one adult take the oldest kid/kids while the other parent does boneyard with the little ones.
Everest is a great coaster, but may be too intense for littles. This might be a rider swap situation.
Festival of the Lion King is a great show, and we loved Nemo, too. Nemo feels more like a broadway type show with storyline and puppets, while Lion King is more spectacle based.
While FOP might be too much for your kids, they'd probably like Navi, if you go at rope drop or have FPs. Many people are reporting the rope drop crowds are going to FOP, leaving Navi an easy walk on at opening.