It sounds like your child has a forearm shaft fracture or as you described it: a break in the middle of the two long bones in the forearm (radius and ulna). A simple fracture without displacement (separation of the bones) can be successfully treated with cast immobilization. But displacement and angulation (bones shift and are no longer straight but instead form a V- or angular-shape) may be severe enough to require surgery.
The decision between cast immobilization and surgical reduction and fixation depends on a number of factors. First, there are guidelines for what is acceptable versus unacceptable angulation and displacement. For example, children under the age of nine can still be treated with cast immobilization when there is complete displacement but only up to 15 degrees of angulation and 45 degrees of malrotation. The reason children with these changes in alignment can still be treated conservatively (nonoperative care) is that they are still growing and the bones will reform, reshape, and realign on their own.
Surgery to realign (reduce) the bones and hold them together with hardware (fixation) is recommended when it would be difficult to keep the fractured bones lined up and held in place with a simple cast. This is often the case when the bones are in a severe V- or angular position referred to as bayonet apposition. Surgery is also required anytime the fracture site is unstable or an acceptable reduction position cannot be reached.
Making the decision to perform surgery is just the beginning of the process. The surgeon will be making other decisions about the type of surgery, technique to use, and whether to perform an open (with incisions) or closed (through the skin) procedure. Not all children need fixation so the challenge becomes the ability to evaluate each child and make the best decision for that patient. Having your pre-approval aids the process so that the surgeon can assess what needs to be done at the time of the surgical procedure.