Reaseyann - If you manage this trip to your abilities and to the abilities of your scouts, you should be fine.
I have backpacked into the canyon 4 times. Each time was different. All good experiences. I have never backpacked there in the summer - mostly March / April.
Judging by the experience level you have indicated, I would stick to the main trails (Rim Trail / Bright Angel Trail / South Kaibab). These are all well travelled and patrolled by rangers.
As someone before mentioned, a hike down the Bright Angel to 1.5 mile resthouse might be doable. Allow about 3 hrs roundtrip. Take at least 3 liters of water per hiker. I believe there's water at the resthouse. This is your turnaround point.
Plenty of sunscreen, loose fitting light colored clothes and a wide-brimmed hat also mandatory. No ballcaps - they don't protect the ears.
Also sunglasses!
There's plenty of checklists if you google "Grand Canyon Day Hike".
http://www.rei.com/expertadvice/articles/backpacking+day+checklist.html?s_kwcid=TC|13029|hike%20checklist||S|p|7499831765&cm_mmc=ps_google_CH-_-Category%20-%20Camp%2fHike-_-Camping%2fHiking_Hiking_Checklist_Advice-_-hike%20Checklist&gclid=CLeJ7s6d6rACFQ9whwodbStRxw
Hope this helps!