I like to consider myself a fairly experienced hiker (although certainly not one who has done the multi-month long trails).
1) From the best that I can tell, they were on the Savage-Lundy trail, not the Hite Cove trail that was reported earlier in this thread. The Savage-Lundy trial is steep and only the turnaround part of it is near any water. It is not easy to just get water if there is a problem. This trail is steep, in full sun during the day, and has no shade due to fires. I haven't done that trail, but from the elevation gain and distance, it looks like it would take me ~4 hours to complete. That is without the difficulties of a baby and a dog slowing me down.
https://www.google.com/maps/dir/37....38,-119.8332702,16z/data=!4m2!4m1!3e2!5m1!1e4
2) Their only water container was 2.5 liters if it was full. That is about what I carry on a typical spring/fall hike. I would carry at least 3 to 5 liters for a summer hike in those conditions. Then my wife would bring her own 3 to 5 liters. This family had too little water for even a single person. And that assumes their water was full from the start.
3a) It is easy to blame them for their stupidity. A dog and a baby should never be out on a long hike like that in full sun and full heat. But also, it looks like they were not prepared with proper emergency gear. I always bring water purification/filtration devices in my emergency pack. I also always bring a bivy sack which could have given them a good amount of shade. They seem to have none of those types of equipment.
3b) I'd like to give them a bit of the benefit of the doubt since we don't know whether any innocent things happened to them. I've been on enough hikes to see many injured people. One simple ankle sprain could have turned that ~4 hour hike that they planned on into a ~10+ hour full sun ordeal. Maybe there was a bear, a wolf, or a rattlesnake on the trail. There usually aren't many ways around an animal on a switchback trail like that. Sometimes, you just have to sit and wait.
4) They also were navigating by phone which really bothers me. A hiking GPS is so, so much better and your lives are worth the cost of a true GPS. Multiple times I've had to turn away from my intended trail and the hiking GPS saved me (usually it is something like adult moose with baby moose, but sometimes it has been geological failures, unstable snow, bison, etc). A hiking GPS will have all the trails nearby, even decades-old fire trails that don't show up online and can't be seen on your phone. A hiking GPS could also show temporary ponds, abandoned toilet pits, or other water sources that the phone would not show.