Yeah, I don't see the problem with it. Generally, the school will take precautions, and make sure parent doesn't teach child, or teach in the same grade, and when it comes to writing the exams (which the child will be sitting), the parent doesn't normally get involved and the onus is on the other teachers.
And when it comes to parent teaching child at home, that's their business, I see nothing wrong with it. After all, if one of my parents were a teacher (at my school or any other), I'd definitely be seeking their help.
Think of the downside too. It may cause embarassment for one of the two, and students may get bullied because of their parents. It's not as great as it's made out to be.