That's a long time for Big Bear in the summer, unless you like to hike and boat. There isn't a whole lot to do there for kids that age, it is a ski town. I would look into renting a boat to go out on the lake perhaps. Also check out the Alpine Slide, but I don't know if your littlest is big enough. There are ziplines, horseback riding, adventure trails, and other things but the 3 year old won't be able to do most of that.