So—if David had the choice, don’t our hypothetical parents who murder their children also have that choice?
Sure, and murdering their child would be Sin.
They can't do it without God Willing to permit it to happen (he could have them struck by lightning instead, if He so Willed), but it's still Sin if they do.
This is really basic, grade-school-level theology. It's disappointing to think that the Satanic doctrines of Arminianism have so infected Protestantism, as to have so eroded the level of even basic theological understanding in the Protestant Church.