I would reach out to local disability lawyers in your area. While most take cases "on contingency" of winning the claim, social security rules do allow attorneys to operate on a "retainer" basis. All attorney fees must be approved by Social Security and at a hearing, there is nothing preventing the attorney from questioning you on you voluntarily and willfully agreeing to the fee structure in this particular case. As for your granddaughter appearing at the hearing, I would say this is imperative. A Judge will certainly want to develop the record by taking the testimony of the allegedly disabled claimant and you, as the grandmother, have an opportunity to offer lay witness testimony. This can be elicited by your attorney, or if you are not represented by an attorney, you can appear at the hearing with your granddaughter and explain to the Judge that you would like to offer lay witness testimony on behalf of your granddaughter. The purpose of it would be to explain how your granddaughter is limited in various ways by her impairments. Good luck.
Any information obtained from this answer should not be treated as legal advice and in no way creates the existance, or appearance, of an attorney-client relationship.
The reason why she was denied when she became an adult is because disability is defined differently for children and adults. You have 60 days from the denial date to appeal. Finding an attorney will not be very difficult because you don't pay anything up front. Fees are agreed to with the fee agreement but the attorney does not get paid until he wins the case. Also, there is only payment if there is back pay and if the Social Security Administration approves the fees.
You can find an attorney here on AVVO. Select "find a lawyer." then type in "social security disability" and your zip code.
I wish you and your granddaughter the best of luck.
If I'm understanding this correctly, she would need too file the next step, should be a request for reconsideration. Attorney fees should be based on a percentage of the back amount. See a social security attorney.