DW and I started taking midday breaks even before DD was born because when we go to Disney we do not do a resort day and we always have at least 7 park days. Even though we are (were) young and in decent shape, that just started taking a toll on our feet more than anything and we discovered that if we took a midday break we got home feeling far less tired than when we didn't. I know some people prefer a resort day instead and I get it but we live in a hot state and both her parents and mine have pools, as well as a neighborhood pool, so we can generally swim pretty easily 8, maybe 9 months out of the year so the pool doesn't hold much of a draw for us. They're nice but not what we are spending a lot of money to travel out of state for.
Since DD was born we discovered that even though she would snooze lightly in her stroller she just did much better getting into our quiet room with AC, cooling off and not having the background noise. The one day that we tried to power through when she was 10 months old and not do a midday break she was an absolute disaster and we had to abandon our Fantasmic seats despite having done the dinner package.
In our experience about an hour and a half to two hours, not counting transit time, is the sweet spot. Everyone cools off, sometimes DW and I nap as well, other times we just quietly talk or read or have a drink on the balcony if the weather is to our liking. DD was age 10 months, 34 months, and 40 months on these trips, so close to your little ones ages. I feel like much longer than that and we actually get more lethargic and are tempted to stay in. While that isn't a bad thing, it's vacation and you should do what you want, we hate the idea of missing too much park time when that's what we came for. Any less and we feel like we are more tired when we get in for the night and almost asking what was the point, other than avoiding the complete meltdown.
Hope that helps, we are in the early planning stages of our next trip ourselves and starting to think about these things again. Whatever you do, enjoy!