Your post indicates you are in San Antonio. If your daughter is on probation out of a Bexar County Court, District or County Court at Law, she will need the permission of the Judge to travel out of state. As a practical matter, it is highly unlikely that a Bexar County Judge will even see, let alone read, a letter from a probationer. She needs to contact her lawyer - I presume she had one when placed on probation. The lawyer will need to file a motion requesting permission to travel, and press to present it to the Judge for approval. The motion will have to include your daughter's itinerary, including where she will be staying, with whom, and contact information at her destination. I agree with the other posts - do not travel without permission. Look at the conditions of probation she was given - such travel is clearly a violation, and could result in a sanction or even revocation.
I agree, contact the probation officer directly and request permission. Generally, the court does not need to approve this request.
Although my intent in answering this question is to aid you in the legal process, my answer does not establish an attorney-client relationship in any way. You should seek the advice and counsel of a qualified attorney in your community to evaluate your legal needs and to advise you. No Attorney-Client Relationship is created without the specific intent of both parties.
It is my understanding from a court liasion probation officer that the probation compact among the states requires that a probationer get permission from their judge to travel out of state. This is why the probation officer or the supervisor themselves cannot simply give the permission. Travel within the state is up to the discretion of the judge - some judges permit travel throughout the state while others restrict travel to their own county and others restrict travel to their own county and continguous counties.
If you do not get an answer immediately from the supervisor, you might talk with a lawyer about talking with the judge about traveling (and the lawyer can explain why he and your daughter are bypassing the supervisor.)
If your daughter does not get the permission, she should NOT travel - it would be a violation of probation and you just never know what might happen.
Although I have answered the question to try to help you, you should consult with a lawyer in your area in person on the matter. In addition, my answer does not establish an attorney-client relationship between us.