I think your plan is fine and you sound like you'll be watching your kids for signs that they need a break, so you should be fine.
Our first trip (me and DD, then 5), we didn't take any breaks. But we also were not in commando or marathon mode. She needed a lot of sleep at night, which meant that we were not at the parks when they opened and never stayed until anywhere near closing, so perhaps we didn't need the break as much as if I'd tried to push her limits. Even so, I let her play in water when we saw that, we did air conditioned rides, and we did pour ice water over our heads sometimes. (OK, we were being silly and I started it to make her laugh, but it helped, lol.) Whether or not you schedule a formal break, it's all about staying within your own child's tolerance level. I really don't look at your schedule and think "Oh my, way too much for any kid!"........