Is it required by law that an parent or guardian attend a probation appointment with a juvenile sentenced to adult probation?

Yes, if a juvenile is being questioned by law enforcement, a parent or guardian would have to provide consent for any interrogation.  However, in this case the facts suggest the minor has already entered a plea or had a trial, which could alter the situation. It sounds like the juvenile is being treated as an adult under the law and that means a parent probably does not have to attend. Does the child have an attorney?  In that case, the attorney would "speak" for the juvenile. Regardless of the situation, it is always important for a parent to be in court and show support for the child. Some judges take this into consideration when determining a sentence.

Because each state has different laws, you should consider contacting an attorney for legal advice for your specific situation. Most bar associations have a referral service where attorneys will provide the first visit free or for a low cost. You could contact your state bar or county bar for assistance.