I have a 9 yo DD and 6 yo DS and I am wondering how much we can do in one day and enjoy it? We have never gone before so I have a hard time imagining the size of the parks and the amount of wait time for rides etc......
When youngest was 4, we went on first trip. It was a whirlwind 4 days and we wanted to see as much as we could. Dkids are early risers. We stuck close to their wake/sleep and eating schedule. I'm a big believer in 3 meals and a morning and afternoon snack for elementary kids.
We would do rope drop (offseason) at 8:00 or 9:00 and go until 7/8ish in the evening. DD would crash out for hour or so nap in the stroller when she was 4.
If they weren't able to handle the sometimes 11-12 hours in the parks we certainly would've altered our schedule. Dkids were happy, agreeable and got their 10-11 hours of sleep at the resort. We do DDiningPlan so we usually had an hour or more of relaxing sitdown time at a restaurant once a day.
Now that kids are a few years older-no naps and an hour or so less sleep a night. We still do ropedrop and usually hit Fantasmic or Wishes (early 7 or 8

o in offseason) before calling it a day. Illuminations is tougher to hit when you do a ropedrop and no resort-nap midday.
All day touring works for us. Kids are happy, we're happy. Though I can imagine a totally different situation in the heat/humidity of summer or if we didn't stick to our eating schedule/routine or make sure they're getting enough sleep.
Prior to WDW trips, we would do family walks of many miles MONTHS before trip, so kids had stamina and endurance for all the walking. We all wear good, sturdy, broken-in already, walking shoes (no flipflops here) and I think that's key too.
We also often park-hop. Some people think that's more effort/tiring. Its sometimes nice to break up a morning of alot of walking or rides with an afternoon with alot of shows that parkhopping allows for.