Child support doesn't get adjusted month to month, neither does custody.
If the amount of time makes a big change to percent over the entire year, then she may have a small point, although even then, it is a temporary situation.
So first, if you have them 50-50 now and this is for two months. If she had them 80% for those two months, it would work out to 55/45% for the year. So from that angle it is crazy.
Second, if you are working more, do the children spend a lot less overnights with you? Or do you pick them up from her later? Is there a way add extra days when you are free to make up? Without knowing your children's schedule and your work schedule it's hard to make a suggestion, but there are a lot of creative ways you could make up the time.
Thirdly, how much extra expense does your ex incur due to the extra time? She has the same housing, the kids won't need a new wardrobe just for those two months, and you are already splitting the cost of activities. So you are primarily looking at groceries, transportation, probably a few things like going out to movies, and a slightly higher utilities bill.
Fourth, do you have any other options? Let's say you put them in daycare or camp for those two months. What would the cost be? If you can work it out, you are within your rights to simply do that and ignore the ex. You have a solid custody schedule, you stick to the schedule.
Take all of this and think it over. If this were me I would say to the ex that this is temporary so the custody doesn't change and if she wants to change it she will have file with the courts. Then offer her some level of compensation and state that this is would cover her extra expenses. Then state that if she doesn't agree, you will keep the kids on your time and arrange child care.
Write it up in a non-aggressive, non-emotional busines-like way. Make clear statements, don't try to justify anything. They are your children, this is your time, you decide what happens. Don't get sucked into a back and forth argument. If you ex disagrees, then say thanks but no thanks, you will deal with it.
When you write something like this, if you try to give a lot of reasons why, you are just inviting an argument. Just state what the offer is and let her say yes or no.