and yet the toddler boys were killed
Yes, toddler boys become young warriors,
and young warriors later become fully man warriors.
Warrior men are a threat to the community survival, since they know the weaknesses, personalities, and the leadership.
Think of the concept of a "Trojan Horse"; it is easier to remove the threat, altogether.
Regarding the 'Mountain Meadows Massacre",
"Vengeance is mine, I will repay, says the Lord." Hebrews 10:30 ESV .
I choose to leave it up to the Lord !