Once, long ago, having been called to appear in court in Scotland as a "Guardian of the Person', I'll suggest that a document more acceptably appreciated than a "notarized letter" can be accomplished by essentially the same type letter officially filed in a local court (in Texas, before the County Judge), recorded, and with you carrying a certified copy of same. Takes only a few moments, minimal filing fee, and the stamps and certifications lend a lot of weight in foreign places, wrapped up as they often are in the trappings of officialdom.