I take scouts into the canyon once a year. Most of my 2nd year boys hike the corridor (S.Kaibab to BA Campground-7.5mi, to Indian Gardens 4.5mi, to BA trailhead 4.5mi).
Some of my 3rd year boys, and most all my 4+ year boys can do the trip outlined in the same 3 days. So your stronger hikers are likely to be bored silly by the overall short distances.
Another consideration of the route you've outlined (or any route for that matter) is water availability. Cottonwood is generally good in the spring, but it has been extremely dry so can't be counted on. Grapevine is listed as perennial, but could be iffy as dry as it has been. Can't count on water anywhere else. Given that, your boys may end up carrying several days worth of water.
A better option(??), I think, is the Hermit to Bright Angel route. The first day would be long (all the way to Hermit campground 7mi) but there is water available at Hermit creek -- MUST FILTER/PURIFY!! -- worst case is a 3mi round trip to the Colorado for water which your stronger boys could do at need. Then about 5 miles to Salt Creek -- water is iffy, but can fill at the halfway point (Monument) rather than carry the entire distance, than another 7 or so to Indian Gardens (reliable water). Then out.
Another route, with zero water concerns would be S. Kaibab to BA Campground-then a layover day with a side trip hike to Ribbon Falls. Then a day up to Indian Gardens. If you get there early can hike out to plateau point and back, then out on day 4. Could also layover at Indian Gardens if you wish to extend the trek and dayhike out to plateau point and maybe over to Horn Creek and back. (nothing spectacular about Horn Creek, but would keep everyone occupied and provide additional experience).
What time of year are you thinking of? Depending on when, if you want the boys to have a 5 night experience could do 3 days in the canyon, then a couple days on nearby Mt Humphreys.
Recommended 3 day trek would be Grandview to Horseshoe Mesa. You would most likely need to make the 2mi round trip water run to the Miner's Bathtub (also a mine shaft to explore along the way), spend a layover day exploring the caves and old mining stuff of Horseshoe Mesa and then hike out the Grandview. Mt Humphrey's is good for an overnight (camping space for a small group at ~11k feet) with option for a summit attempt. My young scouts hiked Mt Humphreys in October. All made it to the 11k camp area by lunch, with some of the boys making a summit attempt (fortunately had 4 adults and we could leave the worn out boys in camp).
Anyway, hope this is not info overload. If you wish, email me directly (firstname.lastname@example.org). I'd be happy to assist with your planning. YIS, Steve