To me Sedona is a tourist trap, but I've never spent a whole day. Maybe you can give some details on the good parts that I must have missed.
It depends on what you're in to. I've been to Sedona a number of times with my ex and probably our favorite was just having an overpriced breakfast at Mesa Grill, which is up by the airport. Well, overpriced if you look just at what the food costs, but well worth the price if you count it as the price of admission to get astounding views in pretty much all directions.
There's also a free overlook with parking lot near the top of the drive up the airport access road. The Jeep tours are also awesome, which I have done too. They are also quite expensive and as an alternative you could just park at one of the hiking trailheads and do some hiking. Based on the ages of the kids involved none of the long hikes will work, but I'm pretty sure there's a few that are only a quarter mile or so to walk up to something pretty from the trailhead parking lot. If the 6-yo is high enough energy, that's still a totally reasonable distance. Walking downtown is kind of fun too and there's a coffee shop with great views, but that probably does nothing for young kids. There's a natural water slide area too, but I'm not sure it will have any water by August. I'm pretty sure all the snowmelt is long since dried up by that late in the year.
Bottom line is that downtown Sedona itself is mainly expensive food, drink, and art galleries. However the
rest of the area is beautiful views, at least one major free visitor center, and even the free vantage lookout from the airport mesa if you skip the restaurant. It's going to be stupid hot in Phoenix in August, so I would not recommend heading into Phx at all, not to mention it's 2-3 hours from Flagstaff and traffic will suck that time of year due to all the Phoenicians going to/from Flagstaff to get out of the heat.
I've lived in south-central AZ my whole life so I actually struggle to think like a tourist, especially since it's been a decade since I had a 6-year old. I'd probably check out TripAdvisor and other tourism websites to get an idea of everything within 100 miles of Flagstaff. I love the idea of ducking in to Bryce Canyon National Park in Utah but that's 5+ hours from Flagstaff and only practical if you're doing it either on the way to or from Vegas. That might make it worth it, but it's still well out of the way versus just taking I-40 from Vegas to Williams/Flagstaff.
OK, now that you have me thinking about it, one of my favorite off the beaten path attractions is on the old Route 66 bypass separate from the I-40. It's the Grand Canyon Caverns. It's a genuine underground cavern, and a pretty decent sized one. You take an elevator straight to the bottom and it's largely level once you're down there, so it should be easy even for young kids. It hardly adds any time versus taking I-40 the whole way, other than the hour or two for the tour of the caverns. Around Williams on the I-40 is Bearizona, which is a bear park. Supposed to be pretty intense; you drive right up close to the bears. I haven't done it personally as it's a newer place, opened just in the last 5-6 years or so.
