Presumption is perhaps not the best word to use. That also I guess depends a lot upon a person's beliefs or point of view. My belief on this came long ago. It started probably with the Amalekites:
"Now go and smite Amalek, and utterly destroy all that they have, and spare them not; but slay both man and woman, infant and suckling, ox and sheep, camel and ass." I Sam 15:3
King Saul messed up but it was not because he spared the infants and sucklings, but because he spared the best of the livestock and perhaps the king. The Amalekites were the enemy and had abused God's people. Their children did not abuse the Israelites, but the unaccountable children suffered for their parents mistake.
And then there was Achan, the Israelites who troubled Israel by holding back silver, garment and wedge of gold contrary to instructions:
"And Joshua, and all Israel with him, took Achan the son of Zerah, and the silver, and the garment, and the wedge of gold, and his sons, and his daughters, and his oxen, and his asses, and his sheep, and his tent, and all that he had: and they brought them unto the valley of Achor.
And Joshua said, Why hast thou troubled us? the LORD shall trouble thee this day. And all Israel stoned him with stones, and burned them with fire, after they had stoned them with stones." Joshua 7:24-25
I notice immediately of course, that his children were stoned with him.
The innocents were killed and it was not the enemy that killed them. Not being accountable themselves due to age, they suffered along with their father, Achan for his mistake.
Old Testament? Certainly, but has God ever changed? Is God not the same yesterday, today and forever? Is God a respecter of persons?
"For I am the LORD, I change not; therefore ye sons of Jacob are not consumed." Mal 3:6
"Every good gift and every perfect gift is from above, and cometh down from the Father of lights, with whom is no variableness, neither shadow of turning." James 1:17
"These things also belong to the wise. It is not good to have respect of persons in judgment." Prov 24:23
"Then Peter opened his mouth, and said, Of a truth I perceive that God is no respecter of persons:" Acts 10:34
Where are the exceptions that allow an under age child to go to God rather than to simply die in his flesh, for spiritually is he not already dead... unless his parents somehow are able to carry him on their salvation?