Hi Nebbish -
'Fraid I'm going to pile on with the others: GC as a day trip from Vegas is a bad idea for all sorts of reasons, one them being the distance to get there and back. It's usually estimated to be abt. 5 hours each way so not really day-trip accessibility. Yes, be sure you're planning for the Grand Canyon National Park, South Rim and NOT Grand Canyon WEST. The latter is an expensive rip-off that, while closer to Vegas, isn't as deep or as "grand" as the park.
Secondly is the light: the peaks and valleys visually flatten in midday sun. The landscape and colors are at their dramatic best earlier and later in the day when the sun is at an angle. You'd miss those golden hours if arriving late and leaving early. BTW, it's not recommended to drive that part of the country in the dark; lots of critters about.
Thirdly is weather: I've been there when Mother Nature didn't cooperate. It wasn't a big issue for us as we always plan multiple nights there but the day trippers were not happy with the rain and fog. BTW, you don't mention what time of year this vacation will be? You can expect snow and ice in winter.
I'll throw in a 4th reason to cut Vegas short? I don't know if you've been there before but the Strip is personally not a place I'd take a child, at least not for more than a few hours. Shoot, that's about all the sleaze my OT and I can handle of it ourselves! Anyway, I'll leave it to you to research and decide but it's not really a family friendly destination, IMHO. If you've not been before, do be aware that your children are not allowed on the casino floors.
So I'd look into spending at least one night, if not two, at the one of the lodges at the South Rim. RESERVE WELL IN ADVANCE. What to do? Just for starters, there are 13 miles of Rim Trail to walk, most of it paved: 8 miles heading west of the Grand Canyon Village to Hermits Rest, and 5 heading east of the village to South Kaibab Trailhead. There are numerous overlooks along the way as well as some historic structures important to the park's history, most, but not all, in the village area. There are also shuttle stops for hopping on should you get tuckered along the way. There are, of course, corridor trails as well (Bright Angel, South Kaibab, etc) that descend below the rim. If not a conditioned hiker, you can choose to do short sections of them, being aware that what goes DOWN must also come UP!
As well there are visitor centers and some of those important structures that I'd mentioned earlier: Kolb Studio, Desert View Watchtower, Hopi House, the train station... Info on all can be found on the park's website. Oh, and the pioneer cemetery has many park-related individuals buried in its hallowed ground:
The 20 miles of Desert View Drive - heading east of the Village out to Desert View overlook - can be driven in your own car. If not coming in the park's East Entrance, which you won't be if coming from Vegas, I'll recommend driving it all the way to the end (Desert View) and then back to the Village, doing the 4 or so overlooks along the way. It can be easier to make righthand hand turns into their parking areas.
You can rent bikes as well, although they must be used only on designated routes:
And lastly, check for organized ranger walks or talks during your stay. They're expected to be authorities on 'their' park and any number of subjects; flora, fauna, indigenous or more recent history, geology, astronomy, etc. Great resources, the rangers.
Obviously, I"m a GC fan! The park's website will be your best resource but hope this helped a little. Please come back with more questions should you have them?