I wouldn't worry about a 3rd child at this point if you aren't pregnant already; you'll have nearly a year's lead-time even if you started TTC tomorrow. Change the agreement when that happens.
Right now, you have two kids, one in school half day, both in school half day in 6 months, and a teacher's schedule.
Do you really always travel on breaks? How does your spouse get all of that time off? What about summers?
In any case, you are looking for someone who values time off more than money, rather than someone looking for the most pay. Then, you negotiate for a slightly lower rate (like $1-$2 lower, not $5 lower) than you might offer if there weren't so many breaks, still pay the guaranteed hours when you're away, and guarantee her those six weeks off every year.
If you think there's any chance that you're going to end up needing/wanting her to work some or all of any of those breaks, then your schedule ceases to be a benefit, even for the time-over-money person.
Or, you can advertise as a teacher if you won't need her in the summer. Some nannies will do this, since plenty of families need full time care in the summer.