3rd conditional plus 2nd conditional?

If you had seen him as a child, you would agree that he is extraordinary.

This is a mixed Conditional. It is not the 3rd Conditional.

3rd Conditional: - The third conditional is used to talk about things which DID NOT HAPPEN in the past.

If you HAD SEEN him as a child, you WOULD HAVE AGREED (then at some point in the past) that he is extraordinary.

Mixed Conditional: -

If you HAD SEEN him as a child, you WOULD AGREE (now at this point) that he is extraordinary

The sentence is fine. Ignore the nonsense about "2nd conditionals" etc and then the sentence won't look problematic any more.