I'm wondering if anyone has experience with Craniosynostosis? Specifically I'm looking for help explaining and preparing my other children for this surgery.
I'm reading your question to say that one of your children is having this surgery, and you're looking for help explaining it to the other children.
The dictionary definition seems helpful, here:
Craniosynostosis [etymology snipped] is a condition in which one or more of the fibrous sutures in an infant skull prematurely fuses by ossification, thereby changing the growth pattern of the skull. Because the skull cannot expand perpendicular to the fused suture, it compensates by growing more in the direction parallel to the closed sutures. Sometimes the resulting growth pattern provides the necessary space for the growing brain, but results in an abnormal head shape and abnormal facial features. In cases in which the compensation does not effectively provide enough space for the growing brain, craniosynostosis results in increased intracranial pressure leading possibly to visual impairment, sleeping impairment, eating difficulties, or an impairment of mental development combined with a significant reduction in IQ.
You can paraphrase that to match your children's age, but just explaining that normal skulls are in separate sections that expand, and what the problem is, and why you want to fix it ought to be a good starting place for a conversation.
Don't be afraid to share facts with your kids! They just want to know stuff. No need to scare them with worst-case-scenarios, but they ought to be able to handle the basics...