I'm not sure what you're trying to say here. Are you suggesting that we should excuse parental abuse, because a child would be a worse parent than an adult?
not at all. Some people are objectively bad parents.
I'm just that in general, the heuristic is accurate that parents have better decision making than their children. The very real counter examples (eg abuse) don't negate the trend.
Put the average child in the position of the average parent and you will see the difference.
Think of how abusive the average parent is, then imagine them trying to parent with 10-20 years less development.