As you are staying off site, you may find it difficult to do a "quick recharge". It takes quite sometime to exit a park (especially MK) and it can take some real effort to be bothered to go back again once you have left!
Our kids are totally different. The eldest (4 -nearly 5) has done 4 trips to WDW and has never needed naps or breaks. She can go on all day if we let her. Our son (2) is the opposite, and gets incredibly grumpy if out for too long without a nap. We also stay offsite, so we found that flexibility was the key to keeping the whole family happy. If we had evening things planned like
MVMCP or a later PS, then we had a low key morning, possibly shopping, and let DS have a nap before venturing out later.
On days when we wanted serious park action, we would get there at opening and stay as long as we were having fun. We usually found 4pm was when the Magic started to wear thin and we left. We never made it back on those days.
As I said before, leaving the park can take well over an hour, sometimes 2 if you staying on I-Drive or further out and real effort is required to go back in again....well, thats what we found anyway! For our family, doing early starts and leaving when we had enough worked well. Evening events and later nights were complemented by light mornings and afternoons. Do what works best for your family. Just remember, if you arent having fun, its not worth forcing yourselves to "do just one more thing". Go home, recharge, relax, come back another day.