Kid Attractions in Las Vegas

Head down to Circus Circus and catch some of the circus acts that go on all day over the casino floors.  There is also the midway arcade and the Adventuredome on the second floor.  This is an amusement park complete with mini golf and a roller coaster ride. 
The Mirage Hotel has the white tiger exhibit and the Secret Garden of Siegfried and Roy.  There is also a dolphin habitat where you can watch the dolphin swim by.
The Excalibur has the medieval pageantry and a whole basement level with arcade and video games.  The Tournament of Kings is one of the nice shows for kids as you get to see a medieval joust and get to eat like they did in King Arthur's days No forks or spoons here, use your fingers.
Gameworks is also a basement level arcade and game center.  It is quite easy to find on the strip, just look for the big Coca-Cola bottle and the M&M shop.  It is located underneath these two shops.  They have batting cages, motion-simulation rides, video games, and air hockey.
For toddlers and young children, you might try the Lied Discovery Children's Museum.  There are plenty of interactive games for them to engage and play.  They could feed a wooden sandwich to a snake and a human to see how they both use nutrition.  There is a mini grocery store, a radio station, and a music/drawing area.  They can also pla basketball in a wheelchair to see how it is to be handicapped.