Matthew Poole and continuators on Genesis 21:7

7 And she said, who would have said upto Abraham, that Sarah should have given children suck ? for I have born him a Son in his old age.

(i. e.) What Man or Woman could believe so improbable a thing? Or, who but a God could have foreseen and foretold it?

She saith Children, though she had but one Child, either by an usual Enallage of the plural number for the singular whereby the word Sons or Daughters is used when there was but one, as Gen. 36. 25. and 46. 23. Numb. 26. 8. or presaging that having received from God a new strength, she might have more Children.

By which expression she sheweth all Mothers what their duty is, viz, to give their Children suck, when they are able to do it, and that neither greatness of quality, nor multitude of business, nor other difficulties and inconveniences, will be a sufficient excuse to those that neglect it.