alesis,
Looks like you have got a lot of good info on your upcoming summer trip to "Red Rock" country. Just a comment on your plans for hiking to the river and back whether a day or backpack trip in the GCNP during July in a few years.
My wife and I have never hiked to the river in July but have been there a number of times in April, early May and have experienced temperatures over 100F while the temperatures on the rim can be quite cool. These are shade temps. The real temps are much higher and shade quite often, is few and far between. I am the first person to encourage families to hike during their vacations and you have said your kids are avid hikers. Only you know their capabilities. Just be aware,The park info does not recommend hiking to the river in the summer.
We have been lucky in the last 5 years to have day hiked or backpacked every official trail in the South Rim, sometimes multiple times, from the Little Colorado to the Boucher Trail so if you Have any questions, just pm me.
Dave