Jacob's descendants were powerless in Egypt. They were mere slaves there. The parent must emphatically tell the child that, from a human perspective, things looked hopeless. But Pharaoh could do nothing against God's mighty arm. He was made powerless and had to let that slave nation go free.
In the rendering of the Fourth Commandment in Deuteronomy 5:15, Moses also pointed out that with a mighty hand and an outstretched arm the Lord your God brought you out.
(We do not find these words in Exodus 20:8–11.) Now that they are on the verge of entering the Promised Land, it is precisely this motivation that should be kept in mind now and forevermore. We too can know that the Lord our God is more powerful than any other power.
21 then you shall say to your son, ‘We were Pharaoh’s slaves in Egypt. And the LORD brought us out of Egypt with a mighty hand.