The mind control goes something like this:
Armageddon is coming soon.
At Armageddon, non-Jehovah's Witnesses and their children get killed.
If one parent is a JW, the kids will be protected until they are old enough to have gotten baptised on their own account.
Once the kids are old enough and should have been baptised, they are potential bird food at Armageddon so anything and anybody who can influence them to not get baptised is a threat to their eternal life playing with vegetarian lions in Paradise.
There is a policy of not revealing truthful information to anyone who doesn't deserve it in their opnion.
Once a child believes that Daddy is influenced by Satan and is going to be killed at Armageddon, it's not a simple matter of informing the kid that Mummy is wrong. The tools the parent needs to have for intelligent debate on this type of subject must be installed in the child before it is indoctrinated, as part of the indoctrination process is teaching the child to turn off the critical thinking abilities that are required for an intervention