I'm not an expert in this area, but I believe the Americans with Disability Act (ADA) provisions apply to 1) employers and 2) public entities (government).
A public entity needs to provide reasonable accommodation unless it can demonstrate doing so would result in:
**A fundamental alteration in the nature of its programs or activities.
**An undue financial or administrative burden.
More info:
www.adaproject.org/gov.html
The PTO is a private, not public, entity. HOWEVER, it might be considered that a PTO meeting is an extension of school programs/activities and that the SCHOOL DISTRICT would be required to supply the interpreter.
www.nad.org/issues/education/k-12/sectio...-and-ada-obligations
If it's true that you are a small organization with minimal budget, you could try explaining that fact and that you don't have funds for an interpreter. Regardless of your legal obligation, what can you do to include the person? Can you ask the school if they have any resources available? Can you find out if they have a family member or friend that might accompany them and interpret? Could you offer to provide extensive notes of the meeting via email (afterwards) so that you communicate back and forth in writing?