Check out what Junior Ranger family programs are going on during your visit. I believe they have some geared toward younger ages. Yosemite Lodge has an ampitheatre with special programs. When you enter the park you will get a schedule of all the activites happening during your stay. Kids also have fun riding the trams. Check out the Indian Village in Yosemite Village.
Happy Isles is a nice little hike and has visitor center. Mirror Meadow/Lake (depending on the time of year), Lower Yosemite Falls, and Bridal Veil Falls are easy hikes. For a little more challenge, go to the Vernal Falls Footbridge (trail head at happy Isles). This is steep but my daughter (age 7) made it. After the footbridge it continues to Vernal falls via a rock staircase that I hiked another time while pregnant. It is difficult but a gorgeous view. With the low rainfall hopefully the waterfalls aren't too dried up. Drive up to Glacier Point and the Mariposa Grove-a must see. Make sure to pack lots of water and snacks. A little first aid kit is good for skinned knees.
There is a full grocery store in Yosemite Village so if you forget something, don't fret. Lots of good places to eat inside the park. The Awahnee Hotel is beautiful to walk around also. Be prepared to take TONS of pictures. Deer walk among the people and bears can be spotted at night. Stellars jay's and squirrels abound.
Yosemite is a wonderful place and you will have a great time.