There are two ways to answer your question: directly and indirectly
Giving suck is an archaic English expression (in use during that time period in England when the KJV was published) that means to feed the child from the mother's breast. We now call that breastfeeding or nursing. (No, there wasn't infant formula in those days). Compare that rendering of the verses with some other translations into English:
New King James Version:
She also said, “Who would have said to Abraham that Sarah would nurse
children? For I have borne him a son in his old age.”
New American Bible:
7 Who would have told Abraham," she added, "that Sarah would nurse
children! Yet I have borne him a son in his old age."
New Revised Standard Version:
‘Who would ever have said to Abraham that Sarah would nurse children?
(Thanks to @StuW, who pointed out that "... the Hebrew reads as the New Revised Standard version.")
This reference to Sarah -- who was long past the usual child bearing age and who was previously barren -- nursing her baby after all of those years of being barren means that she had a baby, so it emphasizes the miraculous nature of Isaac's birth. In that sense, the deeper meaning of that passage is the miracle of Isaac's being her son with Abraham.