Circus Circus has a small amusement park called the Adventure Dome. I remember my parents taking me and my sister to it when we were around 10-12 and we had a lot of fun. There's a roller coaster, a swinging ship, and a waterfall drop ride. There are a lot of carnival type areas too.
You could try to catch the pirate ship battle at Treasure Island, the Mirage volcano eruption, and the singing/dancing Bellagio fountain show. Those shows are all free, but I think they all happen after nightfall so they might not work so well depending on bedtimes.
Make sure you stop at M&Ms World-- it's somewhere I still like to go as an adult. Have a great trip! =)