I hiked down to the river and back in the same day in May of 93. The South Kaibab trail was my route. It is not for the faint of heart. Not that it is scary it is just flat out a hard hike and I am an avid hiker. It is approx. 6 miles down and 6 miles back up. The down is not as easy as it sounds and the up is a lot harder than it sounds. There are parts where the steepness is not as great as other parts but the uphill return portion is steep uphill all the way. The inner gorge is very steep.
My wife and I just hiked down it part way this past mid October. It was 96 deg at Skeleton point (3 miles down)and upper sixties on the rim. There is no water on the South Kaibab trail and very little shade.
I have hiked the Bright Angel trail down to Indian Gardens and on another occasion out to Plateau point. It is not much better it is farther down to the river by a couple miles though I have not taken it all the way to the river. There is water along that trail at several rest areas. Shade is also hard to come by especially once you get down a couple miles. It is more crowded than the South Kaibab as well
When I took the South Kaibab trail down I left at sun up and did not get back to my car until after 9pm it was almost dark. I left to head back up at 10am. I thought I was in good shape but apparently not since it took me 11 hours to get back up six miles.
The South Kaibab trail in my estimation is a much prettier trail to use as you can see much more of the canyon while on it. The Bright Angel is mostly in a major side canyon and you don't get as good views from it though it is still scenic but the other is much better in that regard.
I have grand children that I would love to take there but no way are they going to make it yet. I would suggest doing a portion of either one and see how they do before committing them to such a major undertaking. If you plan on camping you will be carrying a lot of gear. How are you going to carry the youngest at the same time if need be? Minimum a gallon of water for each person on the South Kaibab, you might get away with less on the Bright Angel as there are usually some places to get water but you better check to make sure they are working before heading down as they don't always work.
Not trying to be a naysayer as you are probably fine it is the younger kids who might have the issue and you cant carry them and all the gear.