I think you know there was 9 previous plagues before the 10th or last plague.
( Remember the first born could have been age 30 or older and Not a minor)
Pharaoh hardened his own heart - Exodus 8:15; Exodus 8:32
So, it was the circumstance that brought about Pharaoh hardening his own heart.
Because God 'allowed' Pharaoh to harden his own heart is why (He gave Pharaoh enough rope....., so to speak )
in 'Bible speak' it can be said God hardened the heart of Pharaoh because God allowed it.
As for the parents, many Egyptians went along with Pharaoh according to 1 Samuel 6:6.
Once the violent-world parents of Noah's day were executed, then who would raise their children____________
Parents are responsible for minor children. No parent even put one child on the Ark to be saved !
It wasn't God who used WWII tactics but rather Hitler who used the willing corrupted clergymen -
Nazi photos showing Christian influence