If it was the agent that forgot to add your child - then I would fight tooth and nail over her getting coverage.
What insurance company do you use?
We use BCBS PPO - and it has been a pretty good insruance company for us as far as doctors and hospitals go. The drug coverage company is Caremark - I honestly do not know if that is an arm of BCBS or not.
Also, why did you change insurance agencies? Was it because of a job move? If so, then they HAVE to cover her. But if it was just because you wanted a better rate, you might have a little harder time.
Does the agent you are working with work directly for one of the insurance agencies? If so - he is looking out for what is best for his company.
It might be best to see if you have a local autism advocacy group in your area. I found one by googling "autism support group gwinett county" and found a VERY large one near me that has lots of active people - some even active in legslation here in Georgia.
Also, here is an article that "Autism Speaks" wrote to propose the growing problem of insurance and autism here in America. It is a little lengthy - but it might give you a few ideas of where to get started in your state.
Autism Speaks article on insurance