Is it 'what do your parents have to say about this?' or 'What does your parents have to ssy about this?'
"What DO your parents have to say about this?" is right.
It's similar to these:
1. What DOES John (singular) want? (Not "What DO John want?")
2. What DO John and Jo (plural) want? (Not "What DOES John and Jo want?")
3. What DO we (plural) like? (Not "What DOES we like?")
4. What DO they (plural) feel like eating? (Not "What DOES they feel like eating?")
It's similar to these:
1. What DOES John (singular) want? (Not "What DO John want?")
2. What DO John and Jo (plural) want? (Not "What DOES John and Jo want?")
3. What DO we (plural) like? (Not "What DOES we like?")
4. What DO they (plural) feel like eating? (Not "What DOES they feel like eating?")