Learn English – word meaning “the child’s spouse’s parents”

Wiktionary attests a specific term for the relationship you describe: co-parents-in-law. However, it recommends simply using in-law in conversation:

Rare in conversation, the generic “in-laws” is generally used, with context left to disambiguate. Once grandchildren are born, the term co-grandparents may be used if the focus is on the relationship through the grandchildren rather than though the married couple.

While we most often use in-law to refer to the blood relatives of your own spouse, or sometimes the spouses of your blood relatives, you can also use it for other relatives by marriage. For example, a brother-in-law is your spouse’s brother or your sibling’s husband, and your in-laws are your spouse’s family, but an in-law could be a more distant relation, in context.

If that's unsatisfactory, you can simply say my daughter’s in-laws or my son-in-law’s parents. Indeed, that may be the simplest and clearest way to introduce them.

On a related note: Relationship through marriage is called affinity, as opposed to consanguinuity for blood relations. You can call a relative by marriage affinal kin, although I doubt it’d be understood by anyone but a genealogist or a practitioner of family law.