Pacing: Those that claim it is ambitious are correct, however, we do what we have to do. If that's the time you have and you understand that you will be pushing hard, it will not be a problem.
Temperature: Plan your days well and be willing to get up at sunrise. We were always one of the first people on the trail, and hiked Zion, Arches, GC and others without the heat being a problem. Mid-day, plan to seek refuge and then get out for a few more hours late in the day. If managed closely, you can work with the heat.
Personally, Zion is our favorite NP and I would rob a day from Grand Canyon to add one to Zion. Make sure to get into Watchman when you can!
A trip similar to this changed our family forever. The National Parks are an amazing gift. I hope your family has an experience beyond a vacation!