Welcome to HAZ celticshedevel and "U" and your "h2b" have a awesome but challenging day hike ahead of you. I am sure you and yours have already looked-up the site; however, for my peace of mind I will include it:
http://www.nps.gov/grca/grandcanyon/
*Alright, that said, I'll get on with it. Condition, condition, condition. Find trails which go up hills. Steep hills. I don't know if you guys jog or run, but you might want to consider picking that up if you haven't already. A couple of weeks prior to your Grand Canyon hike, find a challenging trail you have not done and do a inventory hike to see how you both feel afterwards. Above all else, keep this in the back of your mind, no matter what you do, there is no conditioning program which will fully prepare you for a hike in the canyon.
*Have good gear for day hiking and plan to carry water.
*Make sure you get the weather forecast for the day of your hike. Even on a gorgeous, beautiful, fantastic day, the hike is a challenge. Foul weather will only add to that challenge several fold.
*Get to the park early. Say "0 dark thirty", before the sun pops up. Now this is your option, pull all time pieces off and leave them in the car. Time means nothing, time seems to stop climbing out.
*Start down the trail when you can see clear enough. It is amazing when you are below the rim and see the shadows being chased across the canyon and watching rocks change colors during the sunrise.
*If you can, burn the distance/time going down quickly. Minimize sight seeing. After all, you are going down hill to Indian Gardens and then fairly level ground to Plateau Point. Once there, enjoy the view of the river which I believe is another 1000 feet below.
*After the view of the river walk back to indian Gardens kick back about an hour, relax, drink plenty of water, eat some snacks, and watch an occasional backpacker struggling past.
*Start back up to the rim. You can begin to sight see now because you ain't gonna move that fast. Slow, very slow. Take a lot of breaks, you will anyhow. Try not to look up at the trail, you'll see more people struggling just like you, and the switchbacks are always there. They don't seem to disappear as you pass them. The rim still seems to far up. "Could of swore I just past that switchback, dang it!" Conventional time, man-made time means nothing during the climb out, you will be on "Canyon Time", slow, methodical, patient. Just look out over the canyon and marvel at the wonders of nature and her time.
*Once on top, hug each other, pat each other on the back, and treat yourself to a wonderful, well deserve dinner.
In closing, it is a wonderful adventure for a day hike which you will remember forever. Make certain you,re perpared.