Would it be "ethical" to evaluate his fitness to practice a religion?
How about the ethics of suspending driver's licenses?
The answer is surprising. Of course it's ethical. We do it all the time. If someone becomes a felon, he's been evaluated, by definition, as unfit to vote in national elections.
If someone's religion includes human sacrifice, we would prevent him and tell him if he didn't like it to take it up with Quetzacotl (talk about Montezuma's revenge!).
If someone performs egregiously enough behind the wheel, we'll take his license.
Etc., etc., etc.
In each case, the question of whether and how we do what we do hinges on the nature of the deprivation. To inhibit someone's free exercise of his religion is an altogether different matter than to diddle with his driving.
Most Americans seem to devine that parenting is one of those (few) things on the same level of innate fundamentality as voting, or free exercise of religion, or the right to free speech. If we're correct here, then it follows that no matter how elaborate the machinations of the current system may be for evaluating "fitness" unless it can be shown that those machinations are scientifically accurate, there's simply no way for us to do anything other than the harm principle as the way we choose to balance the overwhelming citizen right in question and the legitimate state interest in protecting children from harmful parents.