That this is the same logic as every other merry road to hell, trod with (mostly) good intentions. 
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“You don’t want to indoctrinate your child as (insert religion) – what, you WANT them to be burned in hell for eternity?!”
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“You don’t want surveillance cameras and mics in every room – what, you WANT women and children to be beaten and raped in their own homes, and terrorists to be able to plot without a care in the world?!”
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“You don’t want to have (insert insanely restrictive law about what can be said) – what, you WANT terrorism/hate crimes/genocide?”
Etcetera… 
You duty as a parent is first, stop them having access to phsyical things that are harmful, so no drinking bleach or having unrestricted internet to view porn that’s designed to warp people (to use just two examples), later when they’re old enough to understand you explain why these things are off limits, and what happens to people who consume them, and then you have to let them decide.
I also point out that true freedom arises from encouraging people to observe the truth for themselves; indoctrination implies so curtailing what someone is willing to accept as valid or worthy of study and examination, that they never seek truth because they have been given a deep filter, the doctrine you fed them, designed to keep it out.
And finally, that this is futile overall, though very harmful individually, because if indoctrination worked, none of us would be on here.
It just maims people by telling them whole areas of study are off limits, and provides nothing positive.
Teach them to see, to study, to research without sentiment or desire for approval, and to observe cause and effect. Those are the keys to freedom.
And @anon58416292’s answer is probably the most important practical advice on this topic. 