Will be brutally hot as others have mentioned. May not be family friendly. I’d recommend finding a higher or wetter area. Going up to Bryce or up higher in the Fish Lake NF will definitely offer some better temps. If you’re set on this area, here’s a little advice…
Day 2 isn’t a big day as far as mileage, but you will be climbing up to Observation Point in the sun during the hottest part of the day. Also, there’s no water on the way up, so you’ll possibly be carrying a couple day’s worth. Fill up before you leave the bottom of the main canyon.
On night 2, there is no legal camping immediately around Observation Point. You have to get about half way from Observation Point to the park boundary. It’s completely dry up there too. Camping up there is kind of second rate in my opinion. I’d recommend going straight up to Cable Mountain and Deertrap Mountain.
Hiking from Observation Point area to Cable Mountain and then out on day 3 is a pretty big day in the heat. There is usually pothole water available just above where the trail crosses Echo Canyon (about 10 minutes after the trail split). If it hasn’t rained recently, it gets pretty funky and possibly dries up. There may be water in the Stave Spring area or the drainage west of there, right next to the trail. There’s good water here in the spring and early summer. Haven’t been there in August though. Other than those sources, it will likely be completely dry once you pass Echo Canyon. This area is wide open so you’ll be exposed to the sun all day. I’d recommend taking an umbrella.
If you’re dropping a vehicle on the Ponderosa, I’d recommend stashing some water at both of the Ponderosa trailheads. It would be a little out of the way on days 2 and 3, but would save some heavy water carries.
Enjoy.