Where ever you decide to stay you'll want to check i4vegas.com for rates across all Las Vegas properties. They now list all those hidden "resort fees" so you can see how much more each property is going to charge you per night on top of their listed room rate.

Reading the previous suggestions the Excalibur is the most "kid friendly" and is located on The Strip. The South Point is nicer, but further south. Even more so is the M Casino. But you're going to Vegas to see Vegas, right, so pick a property on The Strip. If you stay off-Strip you'll be driving (and parking) every day. With the rental car fees on top of their rates you're better off staying somewhere that does not require a rental car. Restaurants, shows, casinos, spas, and attractions for the kid are all within walking distance if you stay on The Strip.

Enjoy.