Does anyone know what I'll need? We're leaving Sunday... I can get a certified copy
of our custody papers but the county can't notarize them. I retain 100% legal and physical custody. The Spanish Tourism Board is difficult to reach...

Is this something new, that you need documentation of some sort other than the child's passport and yours? I've travelled dozens of times to Spain alone w/ my son. He and I have the same surname and address on our passports. No one has ever asked questions.

Thank you! I won't worry about the notarization if you haven't had a problem.
General travel advice outside the country is
that you should have a notarized (or certified?) copy of custody papers or if you're married or have joint custody a notarized letter of permission. I could not find any information specific to Spain. This is to prevent abductions. Mexico is particularly strict.

For future reference, I called the Spanish Consulate and they said I did need documentation of custody (certified is fine) or permission (needs to be notarized). To be on the safe side I would bring it.