How is this sentence grammatically incorrect?

Hello
I was made to answer the following question:

Determine whether the section in bold is grammatically correct or incorrect:

Thanks to Bandura and his Bobo doll, modern psychologists are now able to better understand some of the causes behind aggression in children. And parents and teachers can use this information to help reduce youth violence. Although it is difficult to prevent children from watching violent movies or playing violent games, it is important to explain to them that this violence is exaggerated, unrealistic, and immoral. Furthermore, when children use violence to deal with problems, *they should teach that there are always non-violent solutions.*

The answer was 'incorrect'. It probably has to be corrected like this: *they should be taught that there are always non-violent solutions.*
However, since 'parents and teachers' mentioned above, isn't it grammatically correct to accept 'parents and teachers' as 'they'?

Thank you.