Are you staying at the Luxor? If not, with a child i would stay over the road at the MGM. It has a great pool area with a lazy river - "see here las-vegas-hotels.tripadvisor.com/Hotel_Review-g45963-d91891-Reviews-MGM_Grand_Hotel_and_Casino-Las_V egas_Nevada.html"
Also has the monorail running out the back, which is handy for going up the strip. Visit the Hilton for the Star Trek Experience if you are at all geeky like us (grin)
The fountains are great, the Eiffel tower is good too, although maybe a bit expensive - great view of the fountains from the top though (smile)
Fremont Street if you like neon !
Wandering through the big casinos is an eye opener, although beware when out on the strip that there is quite a lot of 'dodgy' material being given out - having a child or partner with you will not stop them either.
In the past few years Las Vegas has swung away from the family image back to the adult playground market. Treasure Island has gone from showing a pirate themed show, to scantily clad sirens - maybe not what you want your 8yo to see (wink)
As for the Canyon, it is stunning and worthy of at least a night. It will take you 5hrs each way.
The "Skywalk www.grandcanyonskywalk.com/" is closer but is not the GCNP, IMO it is a tourist trap.
Hire a car and get out of Vegas for a couple of days. You could see Rte 66 in Flagstaff and the new bridge at "Hoover Dam www.hooverdambypass.org/" should be open in May and Sedona for beautiful scenery, etc.
I shall go now as i have waffled on, sorry (blush)