Potential explanations for this behavior:

1. Confirmation bias
2. The father is quicker to complement the daughter
3. The mother is quicker to criticize the daughter
4. The mother already knows how pretty the hair is because she was the one brushing it
5. The mother was taking pictures of the process and seemed distracted
6. The father participates in less care tasks (brushing hair can uncomfortable and boring and explaining why it’s necessary can be upsetting, so if the mother is taking on all that work, the father gets off the hook implicitly and doesn’t suffer the negative backlash)
7. The father is normally the one who brushes her hair
8. This behavior is modeled by the mother showing off her hair to the father

Note that none of these explanations involve the innate sexual preferences and desires of literal children