Heb:11:20-21 When things were still unseen, they blessed their children. Looking forward positively when things were blurry, when things were uncertain, when future looks bleak, Isaac blessed Jacob and Esau; Jacob blessed Joseph's son's. Isa:54:13. All your children will be taught by the Lord and great will be their peace. Psalm:127:3. Children are a heritage from the Lord. Psalm:128:3. Children will be like olive shoots around your table. It is the Almighty, Sovereign God who promised this and He will not lie.
Heb:11:19 Abraham reasoned that God could raise the dead and figuratively speaking, he did receive Isaac back from death. Who said Christian life is unreasonable? Abraham reasoned based on the One who does things His way. It looks illogical to you because your understanding is limited. Nothing is impossible for God. He is a God who created everything from nothingness. The Sovereign ruler. Abraham believed in this God. And it was natural for him to believe in the normalcy of God. What God could do, he reasoned and therefore he did receive. Believe realistically and logically in the God who is all Powerful. That is reasonable. Look at God not with your finite lenses. Look at the world with His infinitely focussed lens and it will make sense. He is a God who makes impossible things possible. But that is normal for Him. Image credit: https://eccles.utah.edu/news/why-well-reasoned-theories-are-more-important-than-crunched-numbers/