My dd is 10. Here is what she has enjoyed in Las Vegas. Sorry for any duplicates.
M & M World was a huge hit. She really enjoyed looking at all the M & M merchandise. And there is a cute free 3-D film on one of the top floors.
DD loves Despicable Me, so we went to see the Statute of Liberty and the Eiffel Tower.
We love the TV show Tanked and really wanted to see some of their aquariums, but they are basically all gone. We actually went to a couple of locations just to be sure, but it it true - none are left. There is a very nice aquarium at the Silverton, which is free. There is also a permanent shark exhibit at the Mandalay Bay, but it had an admission fee.
My husband and dd love Looney Tunes, and enjoyed the Chuck Jones Exhibit at Circus Circus. I don't think it is worth it full price, but we got heavily discounted tickets on TravelZoo. FWIW, she does not like anything else at Circus Circus and our whole family was really grossed out by the air quality in all parts of the hotel. We all took showers and changed clothes when we got back to our hotel.
I think someone mentioned the Venetian. We enjoyed walking around the canal area inside. This area is just shops and restaurants and is kid friendly. There are "street" performers throughout who are family friendly - musical groups, mimes, singing gondoliers, etc. The hotel sometimes has festivals out front in the palazzo area.
I think someone also mentioned the Bellagio fountain show, which is really great after dark. There is also a conservatory area inside which is decorated seasonally. We were there at Christmas, and dd liked the decorations. There were gingerbread houses, a miniature train, etc.
I am not sure if it is still there, but there used to be an animated fountain area in the shops at Caesar's Palace, with statues which would come to life every half hour or so. The sky over the fountain had simulated changing weather, including accompanied by an indoor rain shower. DD loved it.
If you are into roller coasters, there are several. I think someone mentioned the Stratospere one, but there is also a pretty
amazing one at New York New York which goes in and out of the building. It was a huge hit with the sons of some friends of ours.
There is a free monorail from the Bellagio to I think the Monte Carlo and from Excalibur to Mandalay Bay. When dd was younger she enjoyed riding this. There is also a monorail from the Convention Center to the MGM Grand, but there is a charge and it is not as interesting a ride.
You can try looking on Travelzoo local deals or Living Social or Groupon and seeing what is discounted. That's actually how we found the Chuck Jones experience.