Unfortunately, it happens pretty frequently. Typically, though, adults know pretty quickly when their prescriptions aren't correct. Young children can't tell you.
I would try to stay calm. I would be upset without an apology, but it's true - lawyers are telling companies and doctors to never apologize - it opens the door for litigation as an admission of fault. Obviously, they are the only ones at fault here.
Prescriptions in glasses should always be checked once the glasses are in. Always. However, it was a mistake.
What would make you feel like you were made "whole"? An apology? A refund?
Your child won't have permanent damage from having the wrong prescription for 6 months. Identify what would put the situation "right" for you and write the opthomologist a letter.
If you want an apology, put in writing that you have no intention to sue, that you understand that sometimes procedures break down and mistakes happen, but that you would like an apology and a refund for your lenses.
Updated: I completely get that these glasses are supposed to help your daughter's eyesight improve. I've worn glasses since the 5th grade - too late to make improvements. We still don't know your daughter's age, but if 6 months of bad prescription can change her eyes that quickly, it won't take much to correct it. I understand the process broke down. It sounds like a crappy optical department. If you like the doctor (or he's the only one nearby that's on your insurance), then get the prescriptions from him and have them filled elsewhere. Your prescriptions are YOURS - not his. He cannot charge you for writing out your prescription (at the time) to take elsewhere.