So many comments missing the point.
Regardless of any consideration of who was behind what and whether you take the parents' story at face value or not, the fact is that these paintings were never valued because they were good.
If they're good then it doesn't matter who painted them. People stopped buying when they suspected they may have been the product of a father/daughter duo rather than just the kid. What they had been willing to pay for is not good artwork but for who supposedly made it. The art itself was never of any value to them.
And I'm not saying they have no artistic value, either. What I'm saying is that the very question is irrelevant because buyers in the art world only price and purchase work based on the popularity of the artist, not on the merits of the work. Hell, we see right in the movie that a woman bought Ocean not because she liked it but because it was 'more significant' than the one she liked (and because some relative told her she liked it).
The story of the documentary is one of how the media distorts/creates stories, but its ostensible subject - whether abstract art has any objective value, is still valid. And it's evident from what we see here that any artistic merit had nothing to do with why people sought out, purchased, or touted this kid's paintings.
"I'll book you. I'll book you on something. I'll find something in the book to book you on."