I took my 4 year old nephew here last month. I paid $8.95 for a student Nevada resident ticket and took advantage of August's free kids ticket promotion.
There was a lot to see here, both indoors and outdoors. It was extremely hot (Vegas in August, obviously) so switching between being outside and inside with the AC helped.
My nephew enjoyed all the indoor activities as well as the animals and activities outside. Not a whole lot to see, but for what Vegas offers families and children, this place is worth a visit.