I'm so sorry to hear about your little girl's possible spinal problems. I found a great website for you that explains all about Tethered Spinal Cords in children. The address is
http://neurosurgery4kids.net/tetheredspinalcord.htm. Hopefully it will answer any questions you may have.
I hope the neuroradiologists will be sedating your daughter for the myelogram. It doesn't hurt a whole lot, but it is uncomfortable. You'll have to make sure she stays flat on her back afterwards for a certain amount of time. The radiologist will tell you for how long. This is because myelograms can cause "spinal headaches" in some people if they assume an upright position too quickly. This type of headache is a real killer...I had one many years ago following a myelogram. It's like a super bad migraine. But the headaches can often be prevented by staying flat on one's back and drinking lots of fluids.
I hope everything goes smoothly for your daughter. I know that you must be frightened for her. The prospect of spinal surgery is never an easy one for parents.