The Targum Pseudo-Jonathan on Exodus 13:15 gives the father's answer when the son keeps asking. Why the firstborn? Because of one night.
"When the Word of the Lord had hardened the heart of Pharoh," the Targum paraphrases, "He killed all the firstborn in the land of Mizraim, from the firstborn of man to the firstborn of cattle." The tenth plague was total. Every household in Egypt lost its eldest; every Israelite household kept theirs.
Notice the Targumist's careful phrasing. It is not God directly who hardens Pharaoh's heart but the Memra, the "Word of the Lord." This is a favorite Targumic move: divine action is filtered through the Memra, preserving God's transcendence while still attributing the plague to Him.
"Therefore," the father concludes, "do I sacrifice before the Lord every male that openeth the womb, and every firstborn of my sons I redeem with silver." Two categories, two responses. The firstborn animals are offered. The firstborn sons are bought back. Each act is a small, private echo of the public miracle that saved them.
The Targum wants the ritual to feel unbalanced. You give back something to the God who gave you everything, knowing the debt can never be settled.
Takeaway: gratitude in Judaism is not a feeling. It is a sacrifice you renew every generation.