In my experience, it was not the 4-year old who tired first, at least not if the toddler had a stroller.
Hopping is easy if you are already planning a mid-day break.
Genie+ favors hopping. IMO, if you hop and use G+ effectively, then your odds of more than 8 rides is high, but that might be variable.
August weather though is where you might find a bit of a challenge. Generally, I don't mind the heat most days, but our general strategy is to take a short afternoon break every afternoon during the peak heat/storm hours. In FL, afternoon storms are very common, and they are often absolute downpours. Some days they last 10minutes, other days they last longer. If there is lightning anywhere near WDW, even if it is sunny, then they close the pools, coasters, animal attractions, and stop running any shuttle boats.
What I have found is that (toughing it out through) the afternoon heat tends to make everyone sleepy, especially right after lunch. This is especially true for those who drink alcohol.
Some trips, there are no storms, other times there are storms every day. That could have a big impact on how many rides you are able to cover.
August is also hurricane season. So again, the weather could have a significant impact on your visits.
IMO, the evening weather is often fantastic. People who start early then tough out the afternoons though usually end the day early. Many people do not return after the storms pass.
Much also depends on your general endurance level and health. Some trips everyone is 100%, but there are many trips where oddball things happen and we didn't feel up to marathon days. Plans can get derailed by a blister, soaked sneakers, upset stomach, dehydration, etc.
One more tidbit, WDW considers August to be Halloween season. They have been pushing this back more and more every year. It is pretty crazy! WDW makes more $ when they double sell park admission. I expect they will keep adding more of these moneymakers in the years ahead, and that regular park hours will keep trending down. How many rides you are able to cover will also depends on if you are willing to pay the added PM park fees.
Even with the above, we like WDW in summer. The water rides are especially fun. With rides, we're indoors much of the day. Bay Lake Tower is a great location. A plus is seeing the water parade every night. It is a fun way to end the day.