The new Discovery Children's Museum was great!
It is a new $55M establishment near Charleston Outlet mall. There are too many things to go over, but your little one should be able to burn at least 2-3hrs+ in here!
We really liked the Water World area :
http://www.yelp.com/biz_photos/lied-discovery-childrens-museum-las-vegas?userid=eZZyuJDouIg4p-GYB3PV_A#kaS0mJmbB2-jrPHAPXNlwA
Which had lots of fun stuff like water cannons, dams and waterways, and just fun in the water! They give us cool little vests to keep us "mostly" dry lol. Lots of cool stuff to explore, just keep an eye on your camera in here :)
We also had lots of fun in the 70ft tower! There are tons of little places and activities hiding in each and every nook.
http://www.yelp.com/biz_photos/lied-discovery-childrens-museum-las-vegas?userid=eZZyuJDouIg4p-GYB3PV_A#yCHvVWK1uBl-aDQtasD-2A
Lots of imagination here! My little guy liked exploring so much...every turn had something new to enjoy.
http://www.yelp.com/biz_photos/lied-discovery-childrens-museum-las-vegas?userid=eZZyuJDouIg4p-GYB3PV_A#t-9a2L-cYERl74Ks1ROHoA
Overall, this is a great place to spend some family time. At $12pp (incl kids), it is not cheap, but not expensive either! You can buy yearly passes for the whole family for a little over $100!